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Test mypy plugins, stubs, custom types.

Create a Python file, add comments to lines where you expect mypy to produce an error, run mypy_test, and it will check if actual errors are the same as you expect.


  • Flexible: supports every feature supported by mypy, does not enforce a project structure.
  • Fast: mypy gets run only once for all files at once. Also, no patching, no config generation.
  • Easy to learn: run mypy_test with the same arguments as you would run mypy, and it just works.
  • Lightweight: no dependencies except mypy.
python3 -m pip install mypy-test


  1. Write a file you want to test, add comments to the lines you expect to fail:

    a = 1
    reveal_type(a)  # R:
  2. Run the tool:

    python3 -m mypy_test

Writing the comments

  • The comments have the following format: SEVERITY: MESSAGE.
  • Severity is a one-letter violation severity as reported by mypy.
    • F for "fatal"
    • E for "error"
    • W for "warning"
    • N for "note"
    • R is a shorthand for N: Revealed type is "..."
  • Comment can be on the same line as the violation or on the line before.


var = 1.1
reveal_type(var)  # R: builtins.float

# E: Incompatible types in assignment (expression has type "str", variable has type "float")
var = ""


  • The fastest way to know the severity and the message is to run mypy_test on the code and then copy-paste the resulting message.
  • Make separate functions for every test case, so it can have a nice description and a clean namespace.
  • Place all test files into one directory. For example, /types/ or /tests/types/.


  • pytest-mypy-plugins - pytest plugin, test cases described in a YAML file.
  • pytest-mypy-testing - pytest plugin, tests are described like pytest test cases (but they actually don't get run).