Comparing Python Type Checkers: Typing Spec Conformance

8 hours ago

Python's type system started with PEP 484 and was initially defined by mypy.
Multiple type checkers like Pyright, Pytype, and Pyre emerged, leading to divergent interpretations of typing rules.
The Python typing specification was created to consolidate typing rules and includes a conformance test suite.
The conformance test suite checks for false positives (unexpected errors) and false negatives (missed errors).
As of early March 2026, Pyright leads with a 97.8% pass rate, while mypy lags at 58.3%.
Conformance matters as it affects code compatibility and reduces the need for workarounds.
Conformance does not measure type inference, type refinement, or experimental features.
Other factors like performance, IDE integration, and error messages are also important when choosing a type checker.

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