mypy: Check Python Types Without Running Your Code

Overview

mypy checks Python code against its type hints and reports type errors without executing the program. It helps developers find certain mistakes earlier than runtime testing can, especially in code with annotated functions and variables.

Its gradual typing approach lets teams adopt annotations a piece at a time and retain dynamic typing where it is more convenient. It is useful for individual developers and teams that want type feedback during development or as part of code checks.

Key Features

• Checks Python type hints statically, without running the checked code.
• Supports gradual typing, so annotations can be introduced incrementally.
• Provides type inference, generics, callable types, tuple types, union types, and structural subtyping.
• Runs from the command line against a program or codebase.
• Offers daemon mode for faster incremental checks on large codebases.
• Integrates with editors and tools including VS Code, Vim, Emacs, PyCharm, and pre-commit.
• Can be used in an online playground to try type-checking examples.

Use Cases

• Python developers can catch mismatched values and function arguments while writing annotated code, before running it.
• Teams maintaining large codebases can introduce type hints incrementally and use checks during development.
• Library authors can use static checking to help keep function interfaces and internal code consistent.
• Developers can run the daemon while editing to get quicker feedback on incremental changes.

Getting Started

Install mypy and check a Python program:

python3 -m pip install -U mypy
mypy PROGRAM

See the README and documentation for setup and usage details.

Alternatives

• pyre-check: Pyre is another Python type checker, with incremental checking and Pysa security analysis, but its repository is archived.

Considerations

mypy checks declared or inferred types; it does not execute the program, so static checking is not a substitute for running tests. Its feedback depends on the type information available, and gradual typing means unannotated or dynamically typed areas may receive less static checking. The repository metadata reports the license as NOASSERTION, so verify licensing details before adopting it where that matters.