5. Built-in Types¶
The following sections describe the standard types that are built into the interpreter.
Note
Historically (until release 2.2), Python’s built-in types have differed from user-defined types because it was not possible to use the built-in types as the basis for object-oriented inheritance. This limitation no longer exists.
The principal built-in types are numerics, sequences, mappings, files, classes, instances and exceptions.
Some operations are supported by several object types; in particular, practically all objects can be compared, tested for truth value, and converted to a string (with the repr() function or the slightly different str()
function). The latter function is implicitly used when an object is written by the print()
function.
5.1. Truth Value Testing¶
Any object can be tested for truth value, for use in an if
or while
condition or as operand of the Boolean operations below. The following values are considered false:
None
False
zero of any numeric type, for example,
0
,0L
,0.0
,0j
.any empty sequence, for example,
''
,()
,[]
.any empty mapping, for example,
{}
.instances of user-defined classes, if the class defines a
__nonzero__()
or__len__()
method, when that method returns the integer zero orbool
valueFalse
. 1
All other values are considered true — so objects of many types are always true.
Operations and built-in functions that have a Boolean result always return 0
or False
for false and 1
or True
for true, unless otherwise stated. (Important exception: the Boolean operations or
and and
always return one of their operands.)
5.2. Boolean Operations — and
, or
, not
¶
These are the Boolean operations, ordered by ascending priority:
Operation | Result | Notes |
---|---|---|
| if x is false, then y, else x | (1) |
| if x is false, then x, else y | (2) |
| if x is false, then | (3) |
Notes:
This is a short-circuit operator, so it only evaluates the second argument if the first one is false.
This is a short-circuit operator, so it only evaluates the second argument if the first one is true.
not
has a lower priority than non-Boolean operators, sonot a == b
is interpreted asnot (a == b)
, anda == not b
is a syntax error.
5.3. Comparisons¶
Comparison operations are supported by all objects. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, x < y <= z
is equivalent to x < y and y <= z
, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y
is found to be false).
This table summarizes the comparison operations:
Operation | Meaning | Notes |
---|---|---|
| strictly less than | |
| less than or equal | |
| strictly greater than | |
| greater than or equal | |
| equal | |
| not equal | (1) |
| object identity | |
| negated object identity |
Notes:
!=
can also be written<>
, but this is an obsolete usage kept for backwards compatibility only. New code should always use!=
.
Objects of different types, except different numeric types and different string types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (for example, file objects) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. The <
, <=
, >
and >=
operators will raise a TypeError
exception when any operand is a complex number.
Non-identical instances of a class normally compare as non-equal unless the class defines the __eq__()
method or the __cmp__()
method.
Instances of a class cannot be ordered with respect to other instances of the same class, or other types of object, unless the class defines either enough of the rich comparison methods (__lt__()
, __le__()
, __gt__()
, and __ge__()
) or the __cmp__()
method.
CPython implementation detail: Objects of different types except numbers are ordered by their type names; objects of the same types that don’t support proper comparison are ordered by their address.
Two more operations with the same syntactic priority, in
and not in
, are supported only by sequence types (below).
5.4. Numeric Types — int
, float
, long
, complex
¶
There are four distinct numeric types: plain integers, long integers, floating point numbers, and complex numbers. In addition, Booleans are a subtype of plain integers. Plain integers (also just called integers) are implemented using long
in C, which gives them at least 32 bits of precision (sys.maxint
is always set to the maximum plain integer value for the current platform, the minimum value is -sys.maxint - 1
). Long integers have unlimited precision. Floating point numbers are usually implemented using double
in C; information about the precision and internal representation of floating point numbers for the machine on which your program is running is available in sys.float_info
. Complex numbers have a real and imaginary part, which are each a floating point number. To extract these parts from a complex number z, use z.real
and z.imag
. (The standard library includes additional numeric types, fractions
that hold rationals, and decimal
that hold floating-point numbers with user-definable precision.)
Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including binary, hex, and octal numbers) yield plain integers unless the value they denote is too large to be represented as a plain integer, in which case they yield a long integer. Integer literals with an 'L'
or 'l'
suffix yield long integers ('L'
is preferred because 1l
looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending 'j'
or 'J'
to a numeric literal yields an imaginary number (a complex number with a zero real part) which you can add to an integer or float to get a complex number with real and imaginary parts.
Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the “narrower” type is widened to that of the other, where plain integer is narrower than long integer is narrower than floating point is narrower than complex. Comparisons between numbers of mixed type use the same rule. 2 The constructors int()
, long()
, float()
, and complex()
can be used to produce numbers of a specific type.
All built-in numeric types support the following operations. See The power operator and later sections for the operators’ priorities.
Operation | Result | Notes |
---|---|---|
| sum of x and y | |
| difference of x and y | |
| product of x and y | |
| quotient of x and y | (1) |
| (floored) quotient of x and y | (4)(5) |
| remainder of | (4) |
| x negated | |
| x unchanged | |
| absolute value or magnitude of x | (3) |
| x converted to integer | (2) |
| x converted to long integer | (2) |
| x converted to floating point | (6) |
| a complex number with real part re, imaginary part im. im defaults to zero. | |
| conjugate of the complex number c. (Identity on real numbers) | |
| the pair | (3)(4) |
| x to the power y | (3)(7) |
| x to the power y | (7) |
Notes:
For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value.
Conversion from floats using
int()
orlong()
truncates toward zero like the related function,math.trunc()
. Use the functionmath.floor()
to round downward andmath.ceil()
to round upward.See Built-in Functions for a full description.
Also referred to as integer division. The resultant value is a whole integer, though the result’s type is not necessarily int.
float also accepts the strings “nan” and “inf” with an optional prefix “+” or “-” for Not a Number (NaN) and positive or negative infinity.
New in version 2.6.
Python defines
pow(0, 0)
and0 ** 0
to be1
, as is common for programming languages.
All numbers.Real
types (int
, long
, and float
) also include the following operations:
Operation | Result |
---|---|
x truncated to | |
x rounded to n digits, rounding ties away from zero. If n is omitted, it defaults to 0. | |
the greatest integer as a float <= x | |
the least integer as a float >= x |
5.4.1. Bitwise Operations on Integer Types¶
Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value (this assumes a sufficiently large number of bits that no overflow occurs during the operation).
The priorities of the binary bitwise operations are all lower than the numeric operations and higher than the comparisons; the unary operation ~
has the same priority as the other unary numeric operations (+
and -
).
This table lists the bitwise operations sorted in ascending priority:
Operation | Result | Notes |
---|---|---|
| bitwise or of x and y | |
| bitwise exclusive or of x and y | |
| bitwise and of x and y | |
| x shifted left by n bits | (1)(2) |
| x shifted right by n bits | (1)(3) |
| the bits of x inverted |
Notes:
Negative shift counts are illegal and cause a
ValueError
to be raised.A left shift by n bits is equivalent to multiplication by
pow(2, n)
. A long integer is returned if the result exceeds the range of plain integers.A right shift by n bits is equivalent to division by
pow(2, n)
.
5.4.2. Additional Methods on Integer Types¶
The integer types implement the numbers.Integral
abstract base class. In addition, they provide one more method:
int.
bit_length
()¶
long.
bit_length
()¶Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:
>>> n = -37 >>> bin(n) '-0b100101' >>> n.bit_length() 6
More precisely, if
x
is nonzero, thenx.bit_length()
is the unique positive integerk
such that2**(k-1) <= abs(x) < 2**k
. Equivalently, whenabs(x)
is small enough to have a correctly rounded logarithm, thenk = 1 + int(log(abs(x), 2))
. Ifx
is zero, thenx.bit_length()
returns0
.Equivalent to:
def bit_length(self): s = bin(self) # binary representation: bin(-37) --> '-0b100101' s = s.lstrip('-0b') # remove leading zeros and minus sign return len(s) # len('100101') --> 6
New in version 2.7.
5.4.3. Additional Methods on Float¶
The float type implements the numbers.Real
abstract base class. float also has the following additional methods.
float.
as_integer_ratio
()¶Return a pair of integers whose ratio is exactly equal to the original float and with a positive denominator. Raises
OverflowError
on infinities and aValueError
on NaNs.New in version 2.6.
float.
is_integer
()¶Return
True
if the float instance is finite with integral value, andFalse
otherwise:>>> (-2.0).is_integer() True >>> (3.2).is_integer() False
New in version 2.6.
Two methods support conversion to and from hexadecimal strings. Since Python’s floats are stored internally as binary numbers, converting a float to or from a decimal string usually involves a small rounding error. In contrast, hexadecimal strings allow exact representation and specification of floating-point numbers. This can be useful when debugging, and in numerical work.
float.
hex
()¶Return a representation of a floating-point number as a hexadecimal string. For finite floating-point numbers, this representation will always include a leading
0x
and a trailingp
and exponent.New in version 2.6.
float.
fromhex
(s)¶Class method to return the float represented by a hexadecimal string s. The string s may have leading and trailing whitespace.
New in version 2.6.
Note that float.hex()
is an instance method, while float.fromhex()
is a class method.
A hexadecimal string takes the form:
[sign] ['0x'] integer ['.' fraction] ['p' exponent]
where the optional sign
may by either +
or -
, integer
and fraction
are strings of hexadecimal digits, and exponent
is a decimal integer with an optional leading sign. Case is not significant, and there must be at least one hexadecimal digit in either the integer or the fraction. This syntax is similar to the syntax specified in section 6.4.4.2 of the C99 standard, and also to the syntax used in Java 1.5 onwards. In particular, the output of float.hex()
is usable as a hexadecimal floating-point literal in C or Java code, and hexadecimal strings produced by C’s %a
format character or Java’s Double.toHexString
are accepted by float.fromhex()
.
Note that the exponent is written in decimal rather than hexadecimal, and that it gives the power of 2 by which to multiply the coefficient. For example, the hexadecimal string 0x3.a7p10
represents the floating-point number (3 + 10./16 + 7./16**2) * 2.0**10
, or 3740.0
:
>>> float.fromhex('0x3.a7p10')
3740.0
Applying the reverse conversion to 3740.0
gives a different hexadecimal string representing the same number:
>>> float.hex(3740.0)
'0x1.d380000000000p+11'
5.5. Iterator Types¶
New in version 2.2.
Python supports a concept of iteration over containers. This is implemented using two distinct methods; these are used to allow user-defined classes to support iteration. Sequences, described below in more detail, always support the iteration methods.
One method needs to be defined for container objects to provide iteration support:
container.
__iter__
()¶Return an iterator object. The object is required to support the iterator protocol described below. If a container supports different types of iteration, additional methods can be provided to specifically request iterators for those iteration types. (An example of an object supporting multiple forms of iteration would be a tree structure which supports both breadth-first and depth-first traversal.) This method corresponds to the
tp_iter
slot of the type structure for Python objects in the Python/C API.
The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:
iterator.
__iter__
()¶Return the iterator object itself. This is required to allow both containers and iterators to be used with the
for
andin
statements. This method corresponds to thetp_iter
slot of the type structure for Python objects in the Python/C API.
iterator.
next
()¶Return the next item from the container. If there are no further items, raise the
StopIteration
exception. This method corresponds to thetp_iternext
slot of the type structure for Python objects in the Python/C API.
Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol.
The intention of the protocol is that once an iterator’s next()
method raises StopIteration
, it will continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken. (This constraint was added in Python 2.3; in Python 2.2, various iterators are broken according to this rule.)
5.5.1. Generator Types¶
Python’s generators provide a convenient way to implement the iterator protocol. If a container object’s __iter__()
method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the __iter__()
and next()
methods. More information about generators can be found in the documentation for the yield expression.
5.6. Sequence Types — str
, unicode
, list
, tuple
, bytearray
, buffer
, xrange
¶
There are seven sequence types: strings, Unicode strings, lists, tuples, bytearrays, buffers, and xrange objects.
For other containers see the built in dict
and set
classes, and the collections
module.
String literals are written in single or double quotes: 'xyzzy'
, "frobozz"
. See String literals for more about string literals. Unicode strings are much like strings, but are specified in the syntax using a preceding 'u'
character: u'abc'
, u"def"
. In addition to the functionality described here, there are also string-specific methods described in the String Methods section. Lists are constructed with square brackets, separating items with commas: [a, b, c]
. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, such as a, b, c
or ()
. A single item tuple must have a trailing comma, such as (d,)
.
Bytearray objects are created with the built-in function bytearray()
.
Buffer objects are not directly supported by Python syntax, but can be created by calling the built-in function buffer()
. They don’t support concatenation or repetition.
Objects of type xrange are similar to buffers in that there is no specific syntax to create them, but they are created using the xrange()
function. They don’t support slicing, concatenation or repetition, and using in
, not in
, min()
or max()
on them is inefficient.
Most sequence types support the following operations. The in
and not in
operations have the same priorities as the comparison operations. The +
and *
operations have the same priority as the corresponding numeric operations. 3 Additional methods are provided for Mutable Sequence Types.
This table lists the sequence operations sorted in ascending priority. In the table, s and t are sequences of the same type; n, i and j are integers:
Operation | Result | Notes |
---|---|---|
|
| (1) |
|
| (1) |
| the concatenation of s and t | (6) |
| equivalent to adding s to itself n times | (2) |
| ith item of s, origin 0 | (3) |
| slice of s from i to j | (3)(4) |
| slice of s from i to j with step k | (3)(5) |
| length of s | |
| smallest item of s | |
| largest item of s | |
| index of the first occurrence of x in s | |
| total number of occurrences of x in s |
Sequence types also support comparisons. In particular, tuples and lists are compared lexicographically by comparing corresponding elements. This means that to compare equal, every element must compare equal and the two sequences must be of the same type and have the same length. (For full details see Comparisons in the language reference.)
Notes:
When s is a string or Unicode string object the
in
andnot in
operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. In Python 2.3 and beyond, x may be a string of any length.Values of n less than
0
are treated as0
(which yields an empty sequence of the same type as s). Note that items in the sequence s are not copied; they are referenced multiple times. This often haunts new Python programmers; consider:>>> lists = [[]] * 3 >>> lists [[], [], []] >>> lists[0].append(3) >>> lists [[3], [3], [3]]
What has happened is that
[[]]
is a one-element list containing an empty list, so all three elements of[[]] * 3
are references to this single empty list. Modifying any of the elements oflists
modifies this single list. You can create a list of different lists this way:>>> lists = [[] for i in range(3)] >>> lists[0].append(3) >>> lists[1].append(5) >>> lists[2].append(7) >>> lists [[3], [5], [7]]
Further explanation is available in the FAQ entry How do I create a multidimensional list?.
If i or j is negative, the index is relative to the end of sequence s:
len(s) + i
orlen(s) + j
is substituted. But note that-0
is still0
.The slice of s from i to j is defined as the sequence of items with index k such that
i <= k < j
. If i or j is greater thanlen(s)
, uselen(s)
. If i is omitted orNone
, use0
. If j is omitted orNone
, uselen(s)
. If i is greater than or equal to j, the slice is empty.The slice of s from i to j with step k is defined as the sequence of items with index
x = i + n*k
such that0 <= n < (j-i)/k
. In other words, the indices arei
,i+k
,i+2*k
,i+3*k
and so on, stopping when j is reached (but never including j). When k is positive, i and j are reduced tolen(s)
if they are greater. When k is negative, i and j are reduced tolen(s) - 1
if they are greater. If i or j are omitted orNone
, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k isNone
, it is treated like1
.CPython implementation detail: If s and t are both strings, some Python implementations such as CPython can usually perform an in-place optimization for assignments of the form
s = s + t
ors += t
. When applicable, this optimization makes quadratic run-time much less likely. This optimization is both version and implementation dependent. For performance sensitive code, it is preferable to use thestr.join()
method which assures consistent linear concatenation performance across versions and implementations.Changed in version 2.4: Formerly, string concatenation never occurred in-place.
5.6.1. String Methods¶
Below are listed the string methods which both 8-bit strings and Unicode objects support. Some of them are also available on bytearray
objects.
In addition, Python’s strings support the sequence type methods described in the Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange section. To output formatted strings use template strings or the %
operator described in the String Formatting Operations section. Also, see the re
module for string functions based on regular expressions.
str.
capitalize
()¶Return a copy of the string with its first character capitalized and the rest lowercased.
For 8-bit strings, this method is locale-dependent.
str.
center
(width[, fillchar])¶Return centered in a string of length width. Padding is done using the specified fillchar (default is a space).
Changed in version 2.4: Support for the fillchar argument.
str.
count
(sub[, start[, end]])¶Return the number of non-overlapping occurrences of substring sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.
str.
decode
([encoding[, errors]])¶Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding. errors may be given to set a different error handling scheme. The default is
'strict'
, meaning that encoding errors raiseUnicodeError
. Other possible values are'ignore'
,'replace'
and any other name registered viacodecs.register_error()
, see section Codec Base Classes.New in version 2.2.
Changed in version 2.3: Support for other error handling schemes added.
Changed in version 2.7: Support for keyword arguments added.
str.
encode
([encoding[, errors]])¶Return an encoded version of the string. Default encoding is the current default string encoding. errors may be given to set a different error handling scheme. The default for errors is
'strict'
, meaning that encoding errors raise aUnicodeError
. Other possible values are'ignore'
,'replace'
,'xmlcharrefreplace'
,'backslashreplace'
and any other name registered viacodecs.register_error()
, see section Codec Base Classes. For a list of possible encodings, see section Standard Encodings.New in version 2.0.
Changed in version 2.3: Support for
'xmlcharrefreplace'
and'backslashreplace'
and other error handling schemes added.Changed in version 2.7: Support for keyword arguments added.
str.
endswith
(suffix[, start[, end]])¶Return
True
if the string ends with the specified suffix, otherwise returnFalse
. suffix can also be a tuple of suffixes to look for. With optional start, test beginning at that position. With optional end, stop comparing at that position.Changed in version 2.5: Accept tuples as suffix.
str.
expandtabs
([tabsize])¶Return a copy of the string where all tab characters are replaced by one or more spaces, depending on the current column and the given tab size. Tab positions occur every tabsize characters (default is 8, giving tab positions at columns 0, 8, 16 and so on). To expand the string, the current column is set to zero and the string is examined character by character. If the character is a tab (
\t
), one or more space characters are inserted in the result until the current column is equal to the next tab position. (The tab character itself is not copied.) If the character is a newline (\n
) or return (\r
), it is copied and the current column is reset to zero. Any other character is copied unchanged and the current column is incremented by one regardless of how the character is represented when printed.>>> '01\t012\t0123\t01234'.expandtabs() '01 012 0123 01234' >>> '01\t012\t0123\t01234'.expandtabs(4) '01 012 0123 01234'
str.
find
(sub[, start[, end]])¶Return the lowest index in the string where substring sub is found within the slice
s[start:end]
. Optional arguments start and end are interpreted as in slice notation. Return-1
if sub is not found.
str.
format
(*args, **kwargs)¶Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces
{}
. Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.>>> "The sum of 1 + 2 is {0}".format(1+2) 'The sum of 1 + 2 is 3'
See Format String Syntax for a description of the various formatting options that can be specified in format strings.
This method of string formatting is the new standard in Python 3, and should be preferred to the
%
formatting described in String Formatting Operations in new code.New in version 2.6.
str.
index
(sub[, start[, end]])¶Like
find()
, but raiseValueError
when the substring is not found.
str.
isalnum
()¶Return true if all characters in the string are alphanumeric and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
isalpha
()¶Return true if all characters in the string are alphabetic and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
isdigit
()¶Return true if all characters in the string are digits and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
islower
()¶Return true if all cased characters 4 in the string are lowercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
isspace
()¶Return true if there are only whitespace characters in the string and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
istitle
()¶Return true if the string is a titlecased string and there is at least one character, for example uppercase characters may only follow uncased characters and lowercase characters only cased ones. Return false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
isupper
()¶Return true if all cased characters 4 in the string are uppercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.
join
(iterable)¶Return a string which is the concatenation of the strings in iterable. If there is any Unicode object in iterable, return a Unicode instead. A
TypeError
will be raised if there are any non-string or non Unicode object values in iterable. The separator between elements is the string providing this method.
str.
ljust
(width[, fillchar])¶Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than or equal to
len(s)
.Changed in version 2.4: Support for the fillchar argument.
str.
lower
()¶Return a copy of the string with all the cased characters 4 converted to lowercase.
For 8-bit strings, this method is locale-dependent.
str.
lstrip
([chars])¶Return a copy of the string with leading characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a prefix; rather, all combinations of its values are stripped:>>> ' spacious '.lstrip() 'spacious ' >>> 'www.example.com'.lstrip('cmowz.') 'example.com'
Changed in version 2.2.2: Support for the chars argument.
str.
partition
(sep)¶Split the string at the first occurrence of sep, and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing the string itself, followed by two empty strings.
New in version 2.5.
str.
replace
(old, new[, count])¶Return a copy of the string with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced.
str.
rfind
(sub[, start[, end]])¶Return the highest index in the string where substring sub is found, such that sub is contained within
s[start:end]
. Optional arguments start and end are interpreted as in slice notation. Return-1
on failure.
str.
rindex
(sub[, start[, end]])¶Like
rfind()
but raisesValueError
when the substring sub is not found.
str.
rjust
(width[, fillchar])¶Return the string right justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than or equal to
len(s)
.Changed in version 2.4: Support for the fillchar argument.
str.
rpartition
(sep)¶Split the string at the last occurrence of sep, and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing two empty strings, followed by the string itself.
New in version 2.5.
str.
rsplit
([sep[, maxsplit]])¶Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done, the rightmost ones. If sep is not specified or
None
, any whitespace string is a separator. Except for splitting from the right,rsplit()
behaves likesplit()
which is described in detail below.New in version 2.4.
str.
rstrip
([chars])¶Return a copy of the string with trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a suffix; rather, all combinations of its values are stripped:>>> ' spacious '.rstrip() ' spacious' >>> 'mississippi'.rstrip('ipz') 'mississ'
Changed in version 2.2.2: Support for the chars argument.
str.
split
([sep[, maxsplit]])¶Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done (thus, the list will have at most
maxsplit+1
elements). If maxsplit is not specified or-1
, then there is no limit on the number of splits (all possible splits are made).If sep is given, consecutive delimiters are not grouped together and are deemed to delimit empty strings (for example,
'1,,2'.split(',')
returns['1', '', '2']
). The sep argument may consist of multiple characters (for example,'1<>2<>3'.split('<>')
returns['1', '2', '3']
). Splitting an empty string with a specified separator returns['']
.If sep is not specified or is
None
, a different splitting algorithm is applied: runs of consecutive whitespace are regarded as a single separator, and the result will contain no empty strings at the start or end if the string has leading or trailing whitespace. Consequently, splitting an empty string or a string consisting of just whitespace with aNone
separator returns[]
.For example,
' 1 2 3 '.split()
returns['1', '2', '3']
, and' 1 2 3 '.split(None, 1)
returns['1', '2 3 ']
.
str.
splitlines
([keepends])¶Return a list of the lines in the string, breaking at line boundaries. This method uses the universal newlines approach to splitting lines. Line breaks are not included in the resulting list unless keepends is given and true.
Python recognizes
"\r"
,"\n"
, and"\r\n"
as line boundaries for 8-bit strings.For example:
>>> 'ab c\n\nde fg\rkl\r\n'.splitlines() ['ab c', '', 'de fg', 'kl'] >>> 'ab c\n\nde fg\rkl\r\n'.splitlines(True) ['ab c\n', '\n', 'de fg\r', 'kl\r\n']
Unlike
split()
when a delimiter string sep is given, this method returns an empty list for the empty string, and a terminal line break does not result in an extra line:>>> "".splitlines() [] >>> "One line\n".splitlines() ['One line']
For comparison,
split('\n')
gives:>>> ''.split('\n') [''] >>> 'Two lines\n'.split('\n') ['Two lines', '']
unicode.
splitlines
([keepends])¶Return a list of the lines in the string, like
str.splitlines()
. However, the Unicode method splits on the following line boundaries, which are a superset of the universal newlines recognized for 8-bit strings.Representation
Description
\n
Line Feed
\r
Carriage Return
\r\n
Carriage Return + Line Feed
\v
or\x0b
Line Tabulation
\f
or\x0c
Form Feed
\x1c
File Separator
\x1d
Group Separator
\x1e
Record Separator
\x85
Next Line (C1 Control Code)
\u2028
Line Separator
\u2029
Paragraph Separator
Changed in version 2.7:
\v
and\f
added to list of line boundaries.
str.
startswith
(prefix[, start[, end]])¶Return
True
if string starts with the prefix, otherwise returnFalse
. prefix can also be a tuple of prefixes to look for. With optional start, test string beginning at that position. With optional end, stop comparing string at that position.Changed in version 2.5: Accept tuples as prefix.
str.
strip
([chars])¶Return a copy of the string with the leading and trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a prefix or suffix; rather, all combinations of its values are stripped:>>> ' spacious '.strip() 'spacious' >>> 'www.example.com'.strip('cmowz.') 'example'
Changed in version 2.2.2: Support for the chars argument.
str.
swapcase
()¶Return a copy of the string with uppercase characters converted to lowercase and vice versa.
For 8-bit strings, this method is locale-dependent.
str.
title
()¶Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.
The algorithm uses a simple language-independent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:
>>> "they're bill's friends from the UK".title() "They'Re Bill'S Friends From The Uk"
A workaround for apostrophes can be constructed using regular expressions:
>>> import re >>> def titlecase(s): ... return re.sub(r"[A-Za-z]+('[A-Za-z]+)?", ... lambda mo: mo.group(0)[0].upper() + ... mo.group(0)[1:].lower(), ... s) ... >>> titlecase("they're bill's friends.") "They're Bill's Friends."
For 8-bit strings, this method is locale-dependent.
str.
translate
(table[, deletechars])¶Return a copy of the string where all characters occurring in the optional argument deletechars are removed, and the remaining characters have been mapped through the given translation table, which must be a string of length 256.
You can use the
maketrans()
helper function in thestring
module to create a translation table. For string objects, set the table argument toNone
for translations that only delete characters:>>> 'read this short text'.translate(None, 'aeiou') 'rd ths shrt txt'
New in version 2.6: Support for a
None
table argument.For Unicode objects, the
translate()
method does not accept the optional deletechars argument. Instead, it returns a copy of the s where all characters have been mapped through the given translation table which must be a mapping of Unicode ordinals to Unicode ordinals, Unicode strings orNone
. Unmapped characters are left untouched. Characters mapped toNone
are deleted. Note, a more flexible approach is to create a custom character mapping codec using thecodecs
module (seeencodings.cp1251
for an example).
str.
upper
()¶Return a copy of the string with all the cased characters 4 converted to uppercase. Note that
s.upper().isupper()
might beFalse
ifs
contains uncased characters or if the Unicode category of the resulting character(s) is not “Lu” (Letter, uppercase), but e.g. “Lt” (Letter, titlecase).For 8-bit strings, this method is locale-dependent.
str.
zfill
(width)¶Return the numeric string left filled with zeros in a string of length width. A sign prefix is handled correctly. The original string is returned if width is less than or equal to
len(s)
.New in version 2.2.2.
The following methods are present only on unicode objects:
unicode.
isnumeric
()¶Return
True
if there are only numeric characters in S,False
otherwise. Numeric characters include digit characters, and all characters that have the Unicode numeric value property, e.g. U+2155, VULGAR FRACTION ONE FIFTH.
unicode.
isdecimal
()¶Return
True
if there are only decimal characters in S,False
otherwise. Decimal characters include digit characters, and all characters that can be used to form decimal-radix numbers, e.g. U+0660, ARABIC-INDIC DIGIT ZERO.
5.6.2. String Formatting Operations¶
String and Unicode objects have one unique built-in operation: the %
operator (modulo). This is also known as the string formatting or interpolation operator. Given format % values
(where format is a string or Unicode object), %
conversion specifications in format are replaced with zero or more elements of values. The effect is similar to the using sprintf()
in the C language. If format is a Unicode object, or if any of the objects being converted using the %s
conversion are Unicode objects, the result will also be a Unicode object.
If format requires a single argument, values may be a single non-tuple object. 5 Otherwise, values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary).
A conversion specifier contains two or more characters and has the following components, which must occur in this order:
The
'%'
character, which marks the start of the specifier.Mapping key (optional), consisting of a parenthesised sequence of characters (for example,
(somename)
).Conversion flags (optional), which affect the result of some conversion types.
Minimum field width (optional). If specified as an
'*'
(asterisk), the actual width is read from the next element of the tuple in values, and the object to convert comes after the minimum field width and optional precision.Precision (optional), given as a
'.'
(dot) followed by the precision. If specified as'*'
(an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision.Length modifier (optional).
Conversion type.
When the right argument is a dictionary (or other mapping type), then the formats in the string must include a parenthesised mapping key into that dictionary inserted immediately after the '%'
character. The mapping key selects the value to be formatted from the mapping. For example:
>>> print '%(language)s has %(number)03d quote types.' % \
... {"language": "Python", "number": 2}
Python has 002 quote types.
In this case no *
specifiers may occur in a format (since they require a sequential parameter list).
The conversion flag characters are:
Flag | Meaning |
---|---|
| The value conversion will use the “alternate form” (where defined below). |
| The conversion will be zero padded for numeric values. |
| The converted value is left adjusted (overrides the |
| (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion. |
| A sign character ( |
A length modifier (h
, l
, or L
) may be present, but is ignored as it is not necessary for Python – so e.g. %ld
is identical to %d
.
The conversion types are:
Conversion | Meaning | Notes |
---|---|---|
| Signed integer decimal. | |
| Signed integer decimal. | |
| Signed octal value. | (1) |
| Obsolete type – it is identical to | (7) |
| Signed hexadecimal (lowercase). | (2) |
| Signed hexadecimal (uppercase). | (2) |
| Floating point exponential format (lowercase). | (3) |
| Floating point exponential format (uppercase). | (3) |
| Floating point decimal format. | (3) |
| Floating point decimal format. | (3) |
| Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
| Floating point format. Uses uppercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
| Single character (accepts integer or single character string). | |
| String (converts any Python object using repr()). | (5) |
| String (converts any Python object using | (6) |
| No argument is converted, results in a |
Notes:
The alternate form causes a leading zero (
'0'
) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero.The alternate form causes a leading
'0x'
or'0X'
(depending on whether the'x'
or'X'
format was used) to be inserted before the first digit.The alternate form causes the result to always contain a decimal point, even if no digits follow it.
The precision determines the number of digits after the decimal point and defaults to 6.
The alternate form causes the result to always contain a decimal point, and trailing zeroes are not removed as they would otherwise be.
The precision determines the number of significant digits before and after the decimal point and defaults to 6.
The
%r
conversion was added in Python 2.0.The precision determines the maximal number of characters used.
If the object or format provided is a
unicode
string, the resulting string will also beunicode
.The precision determines the maximal number of characters used.
See PEP 237.
Since Python strings have an explicit length, %s
conversions do not assume that '\0'
is the end of the string.
Changed in version 2.7: %f
conversions for numbers whose absolute value is over 1e50 are no longer replaced by %g
conversions.
Additional string operations are defined in standard modules string
and re
.
5.6.3. XRange Type¶
The xrange
type is an immutable sequence which is commonly used for looping. The advantage of the xrange
type is that an xrange
object will always take the same amount of memory, no matter the size of the range it represents. There are no consistent performance advantages.
XRange objects have very little behavior: they only support indexing, iteration, and the len()
function.
5.6.4. Mutable Sequence Types¶
List and bytearray
objects support additional operations that allow in-place modification of the object. Other mutable sequence types (when added to the language) should also support these operations. Strings and tuples are immutable sequence types: such objects cannot be modified once created. The following operations are defined on mutable sequence types (where x is an arbitrary object):
Operation | Result | Notes |
---|---|---|
| item i of s is replaced by x | |
| slice of s from i to j is replaced by the contents of the iterable t | |
| same as | |
| the elements of | (1) |
| removes the elements of | |
| same as | (2) |
| for the most part the same as | (3) |
| updates s with its contents repeated n times | (11) |
| return number of i’s for which | |
| return smallest k such that | (4) |
| same as | (5) |
| same as | (6) |
| same as | (4) |
| reverses the items of s in place | (7) |
| sort the items of s in place | (7)(8)(9)(10) |
Notes:
t must have the same length as the slice it is replacing.
The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4.
t can be any iterable object.
Raises
ValueError
when x is not found in s. When a negative index is passed as the second or third parameter to theindex()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously,
index()
didn’t have arguments for specifying start and stop positions.When a negative index is passed as the first parameter to the
insert()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously, all negative indices were truncated to zero.
The
pop()
method’s optional argument i defaults to-1
, so that by default the last item is removed and returned.The
sort()
andreverse()
methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don’t return the sorted or reversed list.The
sort()
method takes optional arguments for controlling the comparisons.cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument:
cmp=lambda x,y: cmp(x.lower(), y.lower())
. The default value isNone
.key specifies a function of one argument that is used to extract a comparison key from each list element:
key=str.lower
. The default value isNone
.reverse is a boolean value. If set to
True
, then the list elements are sorted as if each comparison were reversed.In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once. Use
functools.cmp_to_key()
to convert an old-style cmp function to a key function.Changed in version 2.3: Support for
None
as an equivalent to omitting cmp was added.Changed in version 2.4: Support for key and reverse was added.
Starting with Python 2.3, the
sort()
method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).CPython implementation detail: While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python 2.3 and newer makes the list appear empty for the duration, and raises
ValueError
if it can detect that the list has been mutated during a sort.The value n is an integer, or an object implementing
__index__()
. Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained fors * n
under Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.
5.7. Set Types — set
, frozenset
¶
A set object is an unordered collection of distinct hashable objects. Common uses include membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference. (For other containers see the built in dict
, list
, and tuple
classes, and the collections
module.)
New in version 2.4.
Like other collections, sets support x in set
, len(set)
, and for x in set
. Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.
There are currently two built-in set types, set
and frozenset
. The set
type is mutable — the contents can be changed using methods like add()
and remove()
. Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The frozenset
type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.
As of Python 2.7, non-empty sets (not frozensets) can be created by placing a comma-separated list of elements within braces, for example: {'jack', 'sjoerd'}
, in addition to the set
constructor.
The constructors for both classes work the same:
- class
set
([iterable])¶ - class
frozenset
([iterable])¶ Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be
frozenset
objects. If iterable is not specified, a new empty set is returned.Instances of
set
andfrozenset
provide the following operations:len(s)
Return the number of elements in set s (cardinality of s).
x in s
Test x for membership in s.
x not in s
Test x for non-membership in s.
isdisjoint
(other)¶Return
True
if the set has no elements in common with other. Sets are disjoint if and only if their intersection is the empty set.New in version 2.6.
issubset
(other)¶set <= other
Test whether every element in the set is in other.
set < other
Test whether the set is a proper subset of other, that is,
set <= other and set != other
.
issuperset
(other)¶set >= other
Test whether every element in other is in the set.
set > other
Test whether the set is a proper superset of other, that is,
set >= other and set != other
.
union
(*others)¶set | other | ...
Return a new set with elements from the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection
(*others)¶set & other & ...
Return a new set with elements common to the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference
(*others)¶set - other - ...
Return a new set with elements in the set that are not in the others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference
(other)¶set ^ other
Return a new set with elements in either the set or other but not both.
copy
()¶Return a shallow copy of the set.
Note, the non-operator versions of
union()
,intersection()
,difference()
, andsymmetric_difference()
,issubset()
, andissuperset()
methods will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes error-prone constructions likeset('abc') & 'cbs'
in favor of the more readableset('abc').intersection('cbs')
.Both
set
andfrozenset
support set to set comparisons. Two sets are equal if and only if every element of each set is contained in the other (each is a subset of the other). A set is less than another set if and only if the first set is a proper subset of the second set (is a subset, but is not equal). A set is greater than another set if and only if the first set is a proper superset of the second set (is a superset, but is not equal).Instances of
set
are compared to instances offrozenset
based on their members. For example,set('abc') == frozenset('abc')
returnsTrue
and so doesset('abc') in set([frozenset('abc')])
.The subset and equality comparisons do not generalize to a total ordering function. For example, any two non-empty disjoint sets are not equal and are not subsets of each other, so all of the following return
False
:a<b
,a==b
, ora>b
. Accordingly, sets do not implement the__cmp__()
method.Since sets only define partial ordering (subset relationships), the output of the
list.sort()
method is undefined for lists of sets.Set elements, like dictionary keys, must be hashable.
Binary operations that mix
set
instances withfrozenset
return the type of the first operand. For example:frozenset('ab') | set('bc')
returns an instance offrozenset
.The following table lists operations available for
set
that do not apply to immutable instances offrozenset
:update
(*others)¶set |= other | ...
Update the set, adding elements from all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection_update
(*others)¶set &= other & ...
Update the set, keeping only elements found in it and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference_update
(*others)¶set -= other | ...
Update the set, removing elements found in others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference_update
(other)¶set ^= other
Update the set, keeping only elements found in either set, but not in both.
add
(elem)¶Add element elem to the set.
discard
(elem)¶Remove element elem from the set if it is present.
clear
()¶Remove all elements from the set.
Note, the non-operator versions of the
update()
,intersection_update()
,difference_update()
, andsymmetric_difference_update()
methods will accept any iterable as an argument.Note, the elem argument to the
__contains__()
,remove()
, anddiscard()
methods may be a set. To support searching for an equivalent frozenset, a temporary one is created from elem.
See also
- Comparison to the built-in set types
Differences between the
sets
module and the built-in set types.
5.8. Mapping Types — dict
¶
A mapping object maps hashable values to arbitrary objects. Mappings are mutable objects. There is currently only one standard mapping type, the dictionary. (For other containers see the built in list
, set
, and tuple
classes, and the collections
module.)
A dictionary’s keys are almost arbitrary values. Values that are not hashable, that is, values containing lists, dictionaries or other mutable types (that are compared by value rather than by object identity) may not be used as keys. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (such as 1
and 1.0
) then they can be used interchangeably to index the same dictionary entry. (Note however, that since computers store floating-point numbers as approximations it is usually unwise to use them as dictionary keys.)
Dictionaries can be created by placing a comma-separated list of key: value
pairs within braces, for example: {'jack': 4098, 'sjoerd': 4127}
or {4098: 'jack', 4127: 'sjoerd'}
, or by the dict
constructor.
- class
dict
(**kwarg)¶ - class
dict
(mapping, **kwarg) - class
dict
(iterable, **kwarg) Return a new dictionary initialized from an optional positional argument and a possibly empty set of keyword arguments.
If no positional argument is given, an empty dictionary is created. If a positional argument is given and it is a mapping object, a dictionary is created with the same key-value pairs as the mapping object. Otherwise, the positional argument must be an iterable object. Each item in the iterable must itself be an iterable with exactly two objects. The first object of each item becomes a key in the new dictionary, and the second object the corresponding value. If a key occurs more than once, the last value for that key becomes the corresponding value in the new dictionary.
If keyword arguments are given, the keyword arguments and their values are added to the dictionary created from the positional argument. If a key being added is already present, the value from the keyword argument replaces the value from the positional argument.
To illustrate, the following examples all return a dictionary equal to
{"one": 1, "two": 2, "three": 3}
:>>> a = dict(one=1, two=2, three=3) >>> b = {'one': 1, 'two': 2, 'three': 3} >>> c = dict(zip(['one', 'two', 'three'], [1, 2, 3])) >>> d = dict([('two', 2), ('one', 1), ('three', 3)]) >>> e = dict({'three': 3, 'one': 1, 'two': 2}) >>> a == b == c == d == e True
Providing keyword arguments as in the first example only works for keys that are valid Python identifiers. Otherwise, any valid keys can be used.
New in version 2.2.
Changed in version 2.3: Support for building a dictionary from keyword arguments added.
These are the operations that dictionaries support (and therefore, custom mapping types should support too):
len(d)
Return the number of items in the dictionary d.
d[key]
Return the item of d with key key. Raises a
KeyError
if key is not in the map.If a subclass of dict defines a method
__missing__()
and key is not present, thed[key]
operation calls that method with the key key as argument. Thed[key]
operation then returns or raises whatever is returned or raised by the__missing__(key)
call. No other operations or methods invoke__missing__()
. If__missing__()
is not defined,KeyError
is raised.__missing__()
must be a method; it cannot be an instance variable:>>> class Counter(dict): ... def __missing__(self, key): ... return 0 >>> c = Counter() >>> c['red'] 0 >>> c['red'] += 1 >>> c['red'] 1
The example above shows part of the implementation of
collections.Counter
. A different__missing__
method is used bycollections.defaultdict
.New in version 2.5: Recognition of __missing__ methods of dict subclasses.
d[key] = value
Set
d[key]
to value.
del d[key]
Remove
d[key]
from d. Raises aKeyError
if key is not in the map.
key in d
Return
True
if d has a key key, elseFalse
.New in version 2.2.
key not in d
Equivalent to
not key in d
.New in version 2.2.
iter(d)
Return an iterator over the keys of the dictionary. This is a shortcut for
iterkeys()
.
clear
()¶Remove all items from the dictionary.
copy
()¶Return a shallow copy of the dictionary.
fromkeys
(seq[, value])¶Create a new dictionary with keys from seq and values set to value.
fromkeys()
is a class method that returns a new dictionary. value defaults toNone
.New in version 2.3.
get
(key[, default])¶Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to
None
, so that this method never raises aKeyError
.
has_key
(key)¶Test for the presence of key in the dictionary.
has_key()
is deprecated in favor ofkey in d
.
items
()¶Return a copy of the dictionary’s list of
(key, value)
pairs.CPython implementation detail: Keys and values are listed in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions.
If
items()
,keys()
,values()
,iteritems()
,iterkeys()
, anditervalues()
are called with no intervening modifications to the dictionary, the lists will directly correspond. This allows the creation of(value, key)
pairs usingzip()
:pairs = zip(d.values(), d.keys())
. The same relationship holds for theiterkeys()
anditervalues()
methods:pairs = zip(d.itervalues(), d.iterkeys())
provides the same value forpairs
. Another way to create the same list ispairs = [(v, k) for (k, v) in d.iteritems()]
.
iteritems
()¶Return an iterator over the dictionary’s
(key, value)
pairs. See the note fordict.items()
.Using
iteritems()
while adding or deleting entries in the dictionary may raise aRuntimeError
or fail to iterate over all entries.New in version 2.2.
iterkeys
()¶Return an iterator over the dictionary’s keys. See the note for
dict.items()
.Using
iterkeys()
while adding or deleting entries in the dictionary may raise aRuntimeError
or fail to iterate over all entries.New in version 2.2.
itervalues
()¶Return an iterator over the dictionary’s values. See the note for
dict.items()
.Using
itervalues()
while adding or deleting entries in the dictionary may raise aRuntimeError
or fail to iterate over all entries.New in version 2.2.
keys
()¶Return a copy of the dictionary’s list of keys. See the note for
dict.items()
.
pop
(key[, default])¶If key is in the dictionary, remove it and return its value, else return default. If default is not given and key is not in the dictionary, a
KeyError
is raised.New in version 2.3.
popitem
()¶Remove and return an arbitrary
(key, value)
pair from the dictionary.popitem()
is useful to destructively iterate over a dictionary, as often used in set algorithms. If the dictionary is empty, callingpopitem()
raises aKeyError
.
setdefault
(key[, default])¶If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to
None
.
update
([other])¶Update the dictionary with the key/value pairs from other, overwriting existing keys. Return
None
.update()
accepts either another dictionary object or an iterable of key/value pairs (as tuples or other iterables of length two). If keyword arguments are specified, the dictionary is then updated with those key/value pairs:d.update(red=1, blue=2)
.Changed in version 2.4: Allowed the argument to be an iterable of key/value pairs and allowed keyword arguments.
values
()¶Return a copy of the dictionary’s list of values. See the note for
dict.items()
.
viewitems
()¶Return a new view of the dictionary’s items (
(key, value)
pairs). See below for documentation of view objects.New in version 2.7.
viewkeys
()¶Return a new view of the dictionary’s keys. See below for documentation of view objects.
New in version 2.7.
viewvalues
()¶Return a new view of the dictionary’s values. See below for documentation of view objects.
New in version 2.7.
Dictionaries compare equal if and only if they have the same
(key, value)
pairs.
5.8.1. Dictionary view objects¶
The objects returned by dict.viewkeys()
, dict.viewvalues()
and dict.viewitems()
are view objects. They provide a dynamic view on the dictionary’s entries, which means that when the dictionary changes, the view reflects these changes.
Dictionary views can be iterated over to yield their respective data, and support membership tests:
len(dictview)
Return the number of entries in the dictionary.
iter(dictview)
Return an iterator over the keys, values or items (represented as tuples of
(key, value)
) in the dictionary.Keys and values are iterated over in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions. If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond. This allows the creation of
(value, key)
pairs usingzip()
:pairs = zip(d.values(), d.keys())
. Another way to create the same list ispairs = [(v, k) for (k, v) in d.items()]
.Iterating views while adding or deleting entries in the dictionary may raise a
RuntimeError
or fail to iterate over all entries.
x in dictview
Return
True
if x is in the underlying dictionary’s keys, values or items (in the latter case, x should be a(key, value)
tuple).
Keys views are set-like since their entries are unique and hashable. If all values are hashable, so that (key, value) pairs are unique and hashable, then the items view is also set-like. (Values views are not treated as set-like since the entries are generally not unique.) Then these set operations are available (“other” refers either to another view or a set):
dictview & other
Return the intersection of the dictview and the other object as a new set.
dictview | other
Return the union of the dictview and the other object as a new set.
dictview - other
Return the difference between the dictview and the other object (all elements in dictview that aren’t in other) as a new set.
dictview ^ other
Return the symmetric difference (all elements either in dictview or other, but not in both) of the dictview and the other object as a new set.
An example of dictionary view usage:
>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.viewkeys()
>>> values = dishes.viewvalues()
>>> # iteration
>>> n = 0
>>> for val in values:
... n += val
>>> print(n)
504
>>> # keys and values are iterated over in the same order
>>> list(keys)
['eggs', 'bacon', 'sausage', 'spam']
>>> list(values)
[2, 1, 1, 500]
>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> del dishes['sausage']
>>> list(keys)
['spam', 'bacon']
>>> # set operations
>>> keys & {'eggs', 'bacon', 'salad'}
{'bacon'}
5.9. File Objects¶
File objects are implemented using C’s stdio
package and can be created with the built-in open()
function. File objects are also returned by some other built-in functions and methods, such as os.popen()
and os.fdopen()
and the makefile()
method of socket objects. Temporary files can be created using the tempfile
module, and high-level file operations such as copying, moving, and deleting files and directories can be achieved with the shutil
module.
When a file operation fails for an I/O-related reason, the exception IOError
is raised. This includes situations where the operation is not defined for some reason, like seek()
on a tty device or writing a file opened for reading.
Files have the following methods:
file.
close
()¶Close the file. A closed file cannot be read or written any more. Any operation which requires that the file be open will raise a
ValueError
after the file has been closed. Callingclose()
more than once is allowed.As of Python 2.5, you can avoid having to call this method explicitly if you use the
with
statement. For example, the following code will automatically close f when thewith
block is exited:from __future__ import with_statement # This isn't required in Python 2.6 with open("hello.txt") as f: for line in f: print line,
In older versions of Python, you would have needed to do this to get the same effect:
f = open("hello.txt") try: for line in f: print line, finally: f.close()
Note
Not all “file-like” types in Python support use as a context manager for the
with
statement. If your code is intended to work with any file-like object, you can use the functioncontextlib.closing()
instead of using the object directly.
file.
flush
()¶Flush the internal buffer, like
stdio
’sfflush()
. This may be a no-op on some file-like objects.Note
flush()
does not necessarily write the file’s data to disk. Useflush()
followed byos.fsync()
to ensure this behavior.
file.
fileno
()¶Return the integer “file descriptor” that is used by the underlying implementation to request I/O operations from the operating system. This can be useful for other, lower level interfaces that use file descriptors, such as the
fcntl
module oros.read()
and friends.Note
File-like objects which do not have a real file descriptor should not provide this method!
file.
isatty
()¶Return
True
if the file is connected to a tty(-like) device, elseFalse
.Note
If a file-like object is not associated with a real file, this method should not be implemented.
file.
next
()¶A file object is its own iterator, for example
iter(f)
returns f (unless f is closed). When a file is used as an iterator, typically in afor
loop (for example,for line in f: print line.strip()
), thenext()
method is called repeatedly. This method returns the next input line, or raisesStopIteration
when EOF is hit when the file is open for reading (behavior is undefined when the file is open for writing). In order to make afor
loop the most efficient way of looping over the lines of a file (a very common operation), thenext()
method uses a hidden read-ahead buffer. As a consequence of using a read-ahead buffer, combiningnext()
with other file methods (likereadline()
) does not work right. However, usingseek()
to reposition the file to an absolute position will flush the read-ahead buffer.New in version 2.3.
file.
read
([size])¶Read at most size bytes from the file (less if the read hits EOF before obtaining size bytes). If the size argument is negative or omitted, read all data until EOF is reached. The bytes are returned as a string object. An empty string is returned when EOF is encountered immediately. (For certain files, like ttys, it makes sense to continue reading after an EOF is hit.) Note that this method may call the underlying C function
fread()
more than once in an effort to acquire as close to size bytes as possible. Also note that when in non-blocking mode, less data than was requested may be returned, even if no size parameter was given.Note
This function is simply a wrapper for the underlying
fread()
C function, and will behave the same in corner cases, such as whether the EOF value is cached.
file.
readline
([size])¶Read one entire line from the file. A trailing newline character is kept in the string (but may be absent when a file ends with an incomplete line). 6 If the size argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned. When size is not 0, an empty string is returned only when EOF is encountered immediately.
Note
Unlike
stdio
’sfgets()
, the returned string contains null characters ('\0'
) if they occurred in the input.
file.
readlines
([sizehint])¶Read until EOF using
readline()
and return a list containing the lines thus read. If the optional sizehint argument is present, instead of reading up to EOF, whole lines totalling approximately sizehint bytes (possibly after rounding up to an internal buffer size) are read. Objects implementing a file-like interface may choose to ignore sizehint if it cannot be implemented, or cannot be implemented efficiently.
file.
xreadlines
()¶This method returns the same thing as
iter(f)
.New in version 2.1.
Deprecated since version 2.3: Use
for line in file
instead.
file.
seek
(offset[, whence])¶Set the file’s current position, like
stdio
’sfseek()
. The whence argument is optional and defaults toos.SEEK_SET
or0
(absolute file positioning); other values areos.SEEK_CUR
or1
(seek relative to the current position) andos.SEEK_END
or2
(seek relative to the file’s end). There is no return value.For example,
f.seek(2, os.SEEK_CUR)
advances the position by two andf.seek(-3, os.SEEK_END)
sets the position to the third to last.Note that if the file is opened for appending (mode
'a'
or'a+'
), anyseek()
operations will be undone at the next write. If the file is only opened for writing in append mode (mode'a'
), this method is essentially a no-op, but it remains useful for files opened in append mode with reading enabled (mode'a+'
). If the file is opened in text mode (without'b'
), only offsets returned bytell()
are legal. Use of other offsets causes undefined behavior.Note that not all file objects are seekable.
Changed in version 2.6: Passing float values as offset has been deprecated.
file.
tell
()¶Return the file’s current position, like
stdio
’sftell()
.Note
On Windows,
tell()
can return illegal values (after anfgets()
) when reading files with Unix-style line-endings. Use binary mode ('rb'
) to circumvent this problem.
file.
truncate
([size])¶Truncate the file’s size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position. The current file position is not changed. Note that if a specified size exceeds the file’s current size, the result is platform-dependent: possibilities include that the file may remain unchanged, increase to the specified size as if zero-filled, or increase to the specified size with undefined new content. Availability: Windows, many Unix variants.
file.
write
(str)¶Write a string to the file. There is no return value. Due to buffering, the string may not actually show up in the file until the
flush()
orclose()
method is called.
file.
writelines
(sequence)¶Write a sequence of strings to the file. The sequence can be any iterable object producing strings, typically a list of strings. There is no return value. (The name is intended to match
readlines()
;writelines()
does not add line separators.)
Files support the iterator protocol. Each iteration returns the same result as readline()
, and iteration ends when the readline()
method returns an empty string.
File objects also offer a number of other interesting attributes. These are not required for file-like objects, but should be implemented if they make sense for the particular object.
file.
closed
¶bool indicating the current state of the file object. This is a read-only attribute; the
close()
method changes the value. It may not be available on all file-like objects.
file.
encoding
¶The encoding that this file uses. When Unicode strings are written to a file, they will be converted to byte strings using this encoding. In addition, when the file is connected to a terminal, the attribute gives the encoding that the terminal is likely to use (that information might be incorrect if the user has misconfigured the terminal). The attribute is read-only and may not be present on all file-like objects. It may also be
None
, in which case the file uses the system default encoding for converting Unicode strings.New in version 2.3.
file.
errors
¶The Unicode error handler used along with the encoding.
New in version 2.6.
file.
mode
¶The I/O mode for the file. If the file was created using the
open()
built-in function, this will be the value of the mode parameter. This is a read-only attribute and may not be present on all file-like objects.
file.
name
¶If the file object was created using
open()
, the name of the file. Otherwise, some string that indicates the source of the file object, of the form<...>
. This is a read-only attribute and may not be present on all file-like objects.
file.
newlines
¶If Python was built with universal newlines enabled (the default) this read-only attribute exists, and for files opened in universal newline read mode it keeps track of the types of newlines encountered while reading the file. The values it can take are
'\r'
,'\n'
,'\r\n'
,None
(unknown, no newlines read yet) or a tuple containing all the newline types seen, to indicate that multiple newline conventions were encountered. For files not opened in universal newlines read mode the value of this attribute will beNone
.
file.
softspace
¶Boolean that indicates whether a space character needs to be printed before another value when using the
print
statement. Classes that are trying to simulate a file object should also have a writablesoftspace
attribute, which should be initialized to zero. This will be automatic for most classes implemented in Python (care may be needed for objects that override attribute access); types implemented in C will have to provide a writablesoftspace
attribute.
5.10. memoryview type¶
New in version 2.7.
memoryview
objects allow Python code to access the internal data of an object that supports the buffer protocol without copying. Memory is generally interpreted as simple bytes.
- class
memoryview
(obj)¶ Create a
memoryview
that references obj. obj must support the buffer protocol. Built-in objects that support the buffer protocol includestr
andbytearray
(but notunicode
).A
memoryview
has the notion of an element, which is the atomic memory unit handled by the originating object obj. For many simple types such asstr
andbytearray
, an element is a single byte, but other third-party types may expose larger elements.len(view)
returns the total number of elements in the memoryview, view. Theitemsize
attribute will give you the number of bytes in a single element.A
memoryview
supports slicing to expose its data. Taking a single index will return a single element as astr
object. Full slicing will result in a subview:>>> v = memoryview('abcefg') >>> v[1] 'b' >>> v[-1] 'g' >>> v[1:4] <memory at 0x77ab28> >>> v[1:4].tobytes() 'bce'
If the object the memoryview is over supports changing its data, the memoryview supports slice assignment:
>>> data = bytearray('abcefg') >>> v = memoryview(data) >>> v.readonly False >>> v[0] = 'z' >>> data bytearray(b'zbcefg') >>> v[1:4] = '123' >>> data bytearray(b'z123fg') >>> v[2] = 'spam' Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: cannot modify size of memoryview object
Notice how the size of the memoryview object cannot be changed.
memoryview
has two methods:tobytes
()¶Return the data in the buffer as a bytestring (an object of class
str
).>>> m = memoryview("abc") >>> m.tobytes() 'abc'
tolist
()¶Return the data in the buffer as a list of integers.
>>> memoryview("abc").tolist() [97, 98, 99]
There are also several readonly attributes available:
format
¶A string containing the format (in
struct
module style) for each element in the view. This defaults to'B'
, a simple bytestring.
itemsize
¶The size in bytes of each element of the memoryview.
shape
¶A tuple of integers the length of
ndim
giving the shape of the memory as an N-dimensional array.
ndim
¶An integer indicating how many dimensions of a multi-dimensional array the memory represents.
strides
¶A tuple of integers the length of
ndim
giving the size in bytes to access each element for each dimension of the array.
readonly
¶A bool indicating whether the memory is read only.
5.11. Context Manager Types¶
New in version 2.5.
Python’s with
statement supports the concept of a runtime context defined by a context manager. This is implemented using two separate methods that allow user-defined classes to define a runtime context that is entered before the statement body is executed and exited when the statement ends.
The context management protocol consists of a pair of methods that need to be provided for a context manager object to define a runtime context:
contextmanager.
__enter__
()¶Enter the runtime context and return either this object or another object related to the runtime context. The value returned by this method is bound to the identifier in the
as
clause ofwith
statements using this context manager.An example of a context manager that returns itself is a file object. File objects return themselves from __enter__() to allow
open()
to be used as the context expression in awith
statement.An example of a context manager that returns a related object is the one returned by
decimal.localcontext()
. These managers set the active decimal context to a copy of the original decimal context and then return the copy. This allows changes to be made to the current decimal context in the body of thewith
statement without affecting code outside thewith
statement.
contextmanager.
__exit__
(exc_type, exc_val, exc_tb)¶Exit the runtime context and return a Boolean flag indicating if any exception that occurred should be suppressed. If an exception occurred while executing the body of the
with
statement, the arguments contain the exception type, value and traceback information. Otherwise, all three arguments areNone
.Returning a true value from this method will cause the
with
statement to suppress the exception and continue execution with the statement immediately following thewith
statement. Otherwise the exception continues propagating after this method has finished executing. Exceptions that occur during execution of this method will replace any exception that occurred in the body of thewith
statement.The exception passed in should never be reraised explicitly - instead, this method should return a false value to indicate that the method completed successfully and does not want to suppress the raised exception. This allows context management code (such as
contextlib.nested
) to easily detect whether or not an__exit__()
method has actually failed.
Python defines several context managers to support easy thread synchronisation, prompt closure of files or other objects, and simpler manipulation of the active decimal arithmetic context. The specific types are not treated specially beyond their implementation of the context management protocol. See the contextlib
module for some examples.
Python’s generators and the contextlib.contextmanager
decorator provide a convenient way to implement these protocols. If a generator function is decorated with the contextlib.contextmanager
decorator, it will return a context manager implementing the necessary __enter__()
and __exit__()
methods, rather than the iterator produced by an undecorated generator function.
Note that there is no specific slot for any of these methods in the type structure for Python objects in the Python/C API. Extension types wanting to define these methods must provide them as a normal Python accessible method. Compared to the overhead of setting up the runtime context, the overhead of a single class dictionary lookup is negligible.
5.12. Other Built-in Types¶
The interpreter supports several other kinds of objects. Most of these support only one or two operations.
5.12.1. Modules¶
The only special operation on a module is attribute access: m.name
, where m is a module and name accesses a name defined in m’s symbol table. Module attributes can be assigned to. (Note that the import
statement is not, strictly speaking, an operation on a module object; import foo
does not require a module object named foo to exist, rather it requires an (external) definition for a module named foo somewhere.)
A special attribute of every module is __dict__
. This is the dictionary containing the module’s symbol table. Modifying this dictionary will actually change the module’s symbol table, but direct assignment to the __dict__
attribute is not possible (you can write m.__dict__['a'] = 1
, which defines m.a
to be 1
, but you can’t write m.__dict__ = {}
). Modifying __dict__
directly is not recommended.
Modules built into the interpreter are written like this: <module 'sys' (built-in)>
. If loaded from a file, they are written as <module 'os' from '/usr/local/lib/pythonX.Y/os.pyc'>
.
5.12.2. Classes and Class Instances¶
See Objects, values and types and Class definitions for these.
5.12.3. Functions¶
Function objects are created by function definitions. The only operation on a function object is to call it: func(argument-list)
.
There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types.
See Function definitions for more information.
5.12.4. Methods¶
Methods are functions that are called using the attribute notation. There are two flavors: built-in methods (such as append()
on lists) and class instance methods. Built-in methods are described with the types that support them.
The implementation adds two special read-only attributes to class instance methods: m.im_self
is the object on which the method operates, and m.im_func
is the function implementing the method. Calling m(arg-1, arg-2, ..., arg-n)
is completely equivalent to calling m.im_func(m.im_self, arg-1, arg-2, ..., arg-n)
.
Class instance methods are either bound or unbound, referring to whether the method was accessed through an instance or a class, respectively. When a method is unbound, its im_self
attribute will be None
and if called, an explicit self
object must be passed as the first argument. In this case, self
must be an instance of the unbound method’s class (or a subclass of that class), otherwise a TypeError
is raised.
Like function objects, methods objects support getting arbitrary attributes. However, since method attributes are actually stored on the underlying function object (meth.im_func
), setting method attributes on either bound or unbound methods is disallowed. Attempting to set an attribute on a method results in an AttributeError
being raised. In order to set a method attribute, you need to explicitly set it on the underlying function object:
>>> class C:
... def method(self):
... pass
...
>>> c = C()
>>> c.method.whoami = 'my name is method' # can't set on the method
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'instancemethod' object has no attribute 'whoami'
>>> c.method.im_func.whoami = 'my name is method'
>>> c.method.whoami
'my name is method'
See The standard type hierarchy for more information.
5.12.5. Code Objects¶
Code objects are used by the implementation to represent “pseudo-compiled” executable Python code such as a function body. They differ from function objects because they don’t contain a reference to their global execution environment. Code objects are returned by the built-in compile()
function and can be extracted from function objects through their func_code
attribute. See also the code
module.
A code object can be executed or evaluated by passing it (instead of a source string) to the exec
statement or the built-in eval()
function.
See The standard type hierarchy for more information.
5.12.6. Type Objects¶
Type objects represent the various object types. An object’s type is accessed by the built-in function type()
. There are no special operations on types. The standard module types
defines names for all standard built-in types.
Types are written like this: <type 'int'>
.
5.12.7. The Null Object¶
This object is returned by functions that don’t explicitly return a value. It supports no special operations. There is exactly one null object, named None
(a built-in name).
It is written as None
.
5.12.8. The Ellipsis Object¶
This object is used by extended slice notation (see Slicings). It supports no special operations. There is exactly one ellipsis object, named Ellipsis
(a built-in name).
It is written as Ellipsis
. When in a subscript, it can also be written as ...
, for example seq[...]
.
5.12.9. The NotImplemented Object¶
This object is returned from comparisons and binary operations when they are asked to operate on types they don’t support. See Comparisons for more information.
It is written as NotImplemented
.
5.12.10. Boolean Values¶
Boolean values are the two constant objects False
and True
. They are used to represent truth values (although other values can also be considered false or true). In numeric contexts (for example when used as the argument to an arithmetic operator), they behave like the integers 0 and 1, respectively. The built-in function bool()
can be used to convert any value to a Boolean, if the value can be interpreted as a truth value (see section Truth Value Testing above).
They are written as False
and True
, respectively.
5.12.11. Internal Objects¶
See The standard type hierarchy for this information. It describes stack frame objects, traceback objects, and slice objects.
5.13. Special Attributes¶
The implementation adds a few special read-only attributes to several object types, where they are relevant. Some of these are not reported by the dir()
built-in function.
object.
__dict__
¶A dictionary or other mapping object used to store an object’s (writable) attributes.
object.
__methods__
¶Deprecated since version 2.2: Use the built-in function
dir()
to get a list of an object’s attributes. This attribute is no longer available.
object.
__members__
¶Deprecated since version 2.2: Use the built-in function
dir()
to get a list of an object’s attributes. This attribute is no longer available.
instance.
__class__
¶The class to which a class instance belongs.
class.
__bases__
¶The tuple of base classes of a class object.
definition.
__name__
¶The name of the class, type, function, method, descriptor, or generator instance.
The following attributes are only supported by new-style classes.
class.
__mro__
¶This attribute is a tuple of classes that are considered when looking for base classes during method resolution.
class.
mro
()¶This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in
__mro__
.
class.
__subclasses__
()¶Each new-style class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. Example:
>>> int.__subclasses__() [<type 'bool'>]
Footnotes
- 1
Additional information on these special methods may be found in the Python Reference Manual (Basic customization).
- 2
As a consequence, the list
[1, 2]
is considered equal to[1.0, 2.0]
, and similarly for tuples.- 3
They must have since the parser can’t tell the type of the operands.
- 4(1,2,3,4)
Cased characters are those with general category property being one of “Lu” (Letter, uppercase), “Ll” (Letter, lowercase), or “Lt” (Letter, titlecase).
- 5
To format only a tuple you should therefore provide a singleton tuple whose only element is the tuple to be formatted.
- 6
The advantage of leaving the newline on is that returning an empty string is then an unambiguous EOF indication. It is also possible (in cases where it might matter, for example, if you want to make an exact copy of a file while scanning its lines) to tell whether the last line of a file ended in a newline or not (yes this happens!).