mypy_test

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.

Features:

• 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

Usage

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

   a = 1
   reveal_type(a)  # R: builtins.int
   

2. Run the tool:

   python3 -m mypy_test example.py
   

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.

Example:

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

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

Tips:

• 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/.

Alternatives

• 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).