roam/20201007142751-python_typing.org

Python Typing

• Available Type Checkers
• Limitations
• Fun facts
• Relevant PEPs

Python does not require nor enforce strict typing, and instead takes a dynamic approach using Duck Typing.

Gradual Typing support for Python is provided via type hints as described in PEP-484 (which covers Nominal Typing) and expanded upon in PEP-544 (which covers Structural Typing).

Variables, function arguments, and return values can have their types annotated, e.g. "variable x has type integer" or "function y takes a string and returns a list of dictionaries". While these are optional and not checked at run-time, they serve as documentation and can be used by various type checkers to ensure program correctness.

Available Type Checkers

Mypy

http://mypy-lang.org/

Pyright (Microsoft)

https://github.com/Microsoft/pyright

Pyre (Facebook)

https://pyre-check.org/

Pytype (Google)

https://google.github.io/pytype/

Limitations

• Type hints do not guarantee purity.
• Type hints are not checked at run-time.

Fun facts

• Per PEP-285, Python's boolean inherits from int, hence True + True = 2.

Relevant PEPs

PEP	Name	Python Version
3107	Function Annotations	3.0
443	Single-dispatch generic functions	3.4
482	Literature Overview for Type Hints	N/A
483	The Theory of Type Hints	N/A
484	Type Hints	3.5
526	Syntax for Variable Annotations	3.6
544	Protocols: Structural subtyping (static duck typing)	3.8
557	Data Classes	3.7
560	Core support for typing module and generic types	3.7
561	Distributing and Packaging Type Information	3.7
563	Postponed Evaluation of Annotations	3.7
585	Type Hinting Generics In Standard Collections	3.9
586	Literal Types	3.8
589	TypedDict: Type Hints for Dictionaries with a Fixed Set of Keys	3.8
591	Adding a final qualifier to typing	3.8
593	Flexible function and variable annotations	3.9
604	Allow writing union types as X ｜ Y	3.10
612	Parameter Specification Variables	3.10
613	Explicit Type Aliases	3.10
634	Structural Pattern Matching: Specification	3.10
635	Structural Pattern Matching: Motivation and Rationale	3.10
636	Structural Pattern Matching: Tutorial	3.10
638	Syntactic Macros
640	Unused variable syntax	3.10
646	Variadic Generics	3.11
647	User-Defined Type Guards	3.10
649	Deferred Evaluation Of Annotations Using Descriptors	3.13
654	Exception Groups and except*	3.11
655	Marking individual TypedDict items as required or potentially-missing	3.11
673	Self Type	3.11
675	Arbitrary Literal String Type	3.11
692	Using TypedDict for more precise **kwargs typing	3.12
695	Type Parameter Syntax	3.12
698	Override Decorator for Static Typing	3.12