Demystify Python Types for PEP 729 — Kir Chou

PEP 729 aims to establish a new governance model for Python’s type system, endorsed by maintainers of major type checkers (mypy, pytype, pyright, pyre)

Python has multiple type checking approaches:

• CPython interpreter’s runtime type checking
• Static type checkers (mypy, pytype, pyright, pyre)
• Gradual typing system introduced in PEP 484

Different type checkers can produce different results because they:

• Have different implementation approaches
• Handle type inference differently
• Have varying levels of strictness

Type consistency rules in Python:

• Any type is consistent with all other types by default
• Variables without type annotations default to ‘Any’ type
• Type checkers determine if types are compatible at compile time

Main type checker differences:

• mypy: Fastest adoption of new features, good IDE integration
• pytype: Better type inference, higher coverage
• pyright (Microsoft): Focus on IDE integration
• pyre (Meta): Optimized for large codebases

Implementation timeline challenges:

• CPython typically implements new typing features in ~3 months
• Type checkers take 8-21 months to support new features
• This creates a noticeable gap in feature support

Typing system design considerations:

• Based on gradual typing theory
• Allows partial typing of code
• Must balance static checking with runtime performance
• Needs to maintain backwards compatibility

PEP 729 aims to:

• Standardize type checking behavior
• Reduce implementation time gaps
• Improve coordination between type checker implementations
• Create clearer governance for Python’s type system