Check types with mypy

.github/workflows/lint.yml

@@ -2,6 +2,9 @@ name: Lint
 
 on: [push, pull_request, workflow_dispatch]
 
+env:
+  FORCE_COLOR: 1
+
 permissions:
   contents: read
 
@@ -46,3 +49,6 @@ jobs:
       run: tox -e lint
       env:
         PRE_COMMIT_COLOR: always
+
+    - name: Mypy
+      run: tox -e mypy

pyproject.toml

@@ -125,3 +125,34 @@ known-first-party = ["PIL"]
 [tool.pytest.ini_options]
 addopts = "-ra --color=yes"
 testpaths = ["Tests"]
+
+[tool.mypy]
+python_version = "3.8"
+pretty = true
+disallow_any_generics = true
+enable_error_code = "ignore-without-code"
+extra_checks = true
+follow_imports = "silent"
+warn_redundant_casts = true
+warn_unreachable = true
+warn_unused_ignores = true
+exclude = [
+  '^src/PIL/_tkinter_finder.py$',
+  '^src/PIL/DdsImagePlugin.py$',
+  '^src/PIL/FpxImagePlugin.py$',
+  '^src/PIL/Image.py$',
+  '^src/PIL/ImageCms.py$',
+  '^src/PIL/ImageFile.py$',
+  '^src/PIL/ImageFont.py$',
+  '^src/PIL/ImageMath.py$',
+  '^src/PIL/ImageMorph.py$',
+  '^src/PIL/ImageQt.py$',
+  '^src/PIL/ImageShow.py$',
+  '^src/PIL/ImImagePlugin.py$',
+  '^src/PIL/MicImagePlugin.py$',
+  '^src/PIL/PdfParser.py$',
+  '^src/PIL/PyAccess.py$',
+  '^src/PIL/TiffImagePlugin.py$',
+  '^src/PIL/TiffTags.py$',
+  '^src/PIL/WebPImagePlugin.py$',
+]

tox.ini

@@ -29,3 +29,10 @@ pass_env =
 commands =
     pre-commit run --all-files --show-diff-on-failure
     check-manifest
+
+[testenv:mypy]
+skip_install = true
+deps =
+    mypy==1.7.1
+commands =
+    mypy src {posargs}