6.4 KiB
6.4 KiB
Python Typing
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
booleaninherits fromint, henceTrue + 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 |