Cheatmypy Python type checker
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Start here
mypy <file.py>
Check one file
mypy .Check a directory
mypy -p <package>
Check a whole package
mypy --strict .Strictest checks
mypy --ignore-missing-imports .
Silence untyped libs
mypy --install-types
Install missing stubs
x = 1 # type: ignore
Silence one line
dmypy run -- .Fast daemon re-checks
Concepts
- Your code annotations
- Type checker no code runs
- Stubs and py.typed library types
- Errors file:line [code]
Running
mypy <file.py> <dir>
Check files and directories
mypy .Check everything under here
mypy -p <package>
Check a package recursively
mypy -m <module>
Check one module by name
mypy --exclude '<regex>' .
Skip matching paths
mypy --explicit-package-bases .
Fix 'source file found twice'
Needed with src/ layouts and missing __init__.py files; set
mypy_path or namespace_packages in the config as well.
mypy --python-executable <path> .
Use another env's packages
mypy -VPrint the mypy version
Strictness
mypy --strict .Enable the strict flag bundle
Does not include --warn-unreachable. Flags set explicitly
after --strict override it.
mypy --check-untyped-defs .
Check bodies of untyped functions
mypy --disallow-untyped-defs .
Require annotations on functions
mypy --disallow-incomplete-defs .
Forbid half-annotated functions
mypy --disallow-untyped-calls .
Typed code may not call untyped
mypy --disallow-any-generics .
Require type args on list, dict
mypy --warn-return-any .
Warn when returning Any
mypy --warn-unused-ignores .
Flag useless type: ignore
mypy --warn-unreachable .
Flag dead or redundant code
mypy --strict-equality .
Flag impossible == comparisons
Imports and stubs
mypy --ignore-missing-imports .
Silence unresolved imports
Blunt: those modules become Any. Prefer a per-module setting.
mypy --install-types
Install suggested stub packages
mypy --install-types --non-interactive .
Install stubs without asking
pip install types-<name>
Add typeshed stubs for a library
Output
mypy --pretty .Show source snippets and carets
mypy --show-error-codes .
Print the [code] of each error
mypy --no-color-output .
Plain text for logs
Config
mypy --config-file <path> .
Use a specific config file
mypy --warn-unused-configs .
Catch misspelled sections
mypy --disable-error-code <code> .
Turn off one error kind
| File | Section |
|---|---|
mypy.ini |
[mypy] |
.mypy.ini |
[mypy] |
pyproject.toml |
[tool.mypy] |
setup.cfg |
[mypy] |
Ignoring and annotating
x = f() # type: ignore
Suppress errors on this line
x = f() # type: ignore[<code>]
Suppress only one error code
# mypy: ignore-errors
Skip the whole file (top comment)
reveal_type(<expr>)
Print the inferred type
Only for mypy runs; remove it afterwards. At runtime it fails
(Python 3.11+ has typing.reveal_type).
assert isinstance(<x>, <type>)
Narrow a type
if TYPE_CHECKING:
Imports only for the checker
def f(x: int | None) -> str:
Annotate args and return
Daemon and cache
dmypy run -- <args>
Check, starting the daemon if needed
dmypy recheckRe-check changed files only
dmypy statusShow whether the daemon runs
dmypy stopStop the daemon politely
rm -rf .mypy_cache
Reset after odd results
Recipes
[tool.mypy]python_version = "3.12"strict = truewarn_unreachable = true[[tool.mypy.overrides]]module = ["yaml.*", "requests_toolbelt.*"]ignore_missing_imports = true# start loose, tighten per package[tool.mypy]check_untyped_defs = true[[tool.mypy.overrides]]module = "myapp.core.*"disallow_untyped_defs = truepip install mypy types-requestsmypy --warn-unused-configs --pretty -p myappdmypy run -- --pretty src/# edit code, run againdmypy run -- --pretty src/dmypy stop
