Python Type Checker Comparison: Evaluating Conformance to Typing Specifications
By
ocamoss
Hand-rolled, kettle-boiled, baked to perfection. Worth every minute at the bakery.
Summary
This article examines how well different Python type checkers conform to the official Python typing specification. It discusses the history of Python's type system starting with PEP 484 and mypy as the de-facto standard, then analyzes the conformance status of various type checkers including Pyrefly, Ty, Pyright, and Mypy. The article explains what typing spec conformance means, why it matters for developers, and what limitations exist in current conformance measurements.
Key quotes
· 3 pulledWhen you write typed Python, you expect your type checker to follow the rules of the language. But how closely do today's type checkers actually follow the Python typing specification?
Python's type system started with PEP 484. At the time, the semantics of the type system were mostly defined by the reference implementation, mypy. In practice, whatever mypy implemented became the de-facto specification.
Learn what it means to conform to the Python typing spec, why it matters, and the conformance status of each type checker including Pyrefly, Ty, Pyright and Mypy.
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