Designing a State Machine in Python for 2025: A Clear Introduction

Aleksei Aleinikov

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May 21, 2025

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Every software object lives through phases. A library book can be “On Shelf”, “Reserved”, “Borrowed”, “Overdue”; a pizza order goes “Created”, “Cooking”, “Delivering”, “Delivered”.
Making those phases explicit wipes out cryptic conditionals and boosts readability.

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State machines without scary math

• Finite set of states — their number is limited.
• Exactly one active state at any given moment.
• Strict transition rules define legal moves.

Fun fact: regex engines use state machines internally, which explains their speed.

A “bad” snippet with implicit state

if (book.borrower and
    (datetime.now() - book.borrow_date).days > 30) or \
   (book.reserved_by and book.is_overdue()):
    notify_librarian(book)

What happens here

• We check borrower.
• Compute days since borrow_date.
• Plus an extra branch with reserved_by and is_overdue().
  All we really need is “book is overdue”, yet it’s buried in logic.

The same check after making state explicit