Turn on mypy type-checking in CI

mypy.ini

@@ -13,30 +13,21 @@ exclude = (?x)(
 	| ^setuptools/_distutils/ # Vendored
 	| ^setuptools/config/_validate_pyproject/ # Auto-generated
 	)
-disable_error_code = 
-	# TODO: Test environment is not yet properly configured to install all imported packages
-	# import-not-found, # This can be left commented out for local runs until we enforce running mypy in the CI
-	# TODO: Not all dependencies are typed. Namely: distutils._modified, wheel.wheelfile, and jaraco.*
-	import-untyped,
-	# Ignoring attr-defined because setuptools wraps a lot of distutils classes, adding new attributes,
-	# w/o updating all the attributes and return types from the base classes for type-checkers to understand
-	# Especially with setuptools.dist.command vs distutils.dist.command vs setuptools._distutils.dist.command
-	# *.extern modules that actually live in *._vendor will also cause attr-defined issues on import
-	attr-defined,
+# Ignoring attr-defined because setuptools wraps a lot of distutils classes, adding new attributes,
+# w/o updating all the attributes and return types from the base classes for type-checkers to understand
+# Especially with setuptools.dist.command vs distutils.dist.command vs setuptools._distutils.dist.command
+# *.extern modules that actually live in *._vendor will also cause attr-defined issues on import
+disable_error_code = attr-defined
 
-# Avoid raising issues when importing from "extern" modules, as those are added to path dynamically.
-# https://github.com/pypa/setuptools/pull/3979#discussion_r1367968993
-[mypy-pkg_resources.extern.*,setuptools.extern.*]
+# - Avoid raising issues when importing from "extern" modules, as those are added to path dynamically.
+#   https://github.com/pypa/setuptools/pull/3979#discussion_r1367968993
+# - distutils._modified has different errors on Python 3.8 [import-untyped], on Python 3.9+ [import-not-found]
+# - All jaraco modules are still untyped
+[mypy-pkg_resources.extern.*,setuptools.extern.*,distutils._modified,jaraco.*]
 ignore_missing_imports = True
 
-[mypy-pkg_resources.tests.*,setuptools.tests.*]
-disable_error_code =
-	# Tests include creating dynamic modules that won't exists statically before the test is run.
-	# Let's ignore all "import-not-found", as if an import really wasn't found, then the test would fail.
-	import-not-found,
-	# mmany untyped "jaraco" modules
-	import-untyped,
-
-# Mypy issue, this vendored module is already excluded!
-[mypy-setuptools._vendor.packaging._manylinux]
+# - pkg_resources tests create modules that won't exists statically before the test is run.
+#   Let's ignore all "import-not-found" since, if an import really wasn't found, then the test would fail.
+# - setuptools._vendor.packaging._manylinux: Mypy issue, this vendored module is already excluded!
+[mypy-pkg_resources.tests.*,setuptools._vendor.packaging._manylinux]
 disable_error_code = import-not-found

pyproject.toml

@@ -4,6 +4,3 @@ build-backend = "setuptools.build_meta"
 backend-path = ["."]
 
 [tool.setuptools_scm]
-
-[tool.pytest-enabler.mypy]
-# disabled

setup.cfg

@@ -47,9 +47,7 @@ testing =
 	pytest-cov; \
 		# coverage seems to make PyPy extremely slow
 		python_implementation != "PyPy"
-	pytest-mypy >= 0.9.1; \
-		# workaround for jaraco/skeleton#22
-		python_implementation != "PyPy"
+	pytest-mypy
 	pytest-enabler >= 2.2
 	# workaround for pypa/setuptools#3921
 	pytest-ruff >= 0.2.1; sys_platform != "cygwin"
@@ -66,13 +64,16 @@ testing =
 	filelock>=3.4.0
 	ini2toml[lite]>=0.9
 	tomli-w>=1.0.0
+	tomli
 	pytest-timeout
 	pytest-perf; \
 		# workaround for jaraco/inflect#195, pydantic/pydantic-core#773 (see #3986)
 		sys_platform != "cygwin"
 	# for tools/finalize.py
 	jaraco.develop >= 7.21; python_version >= "3.9" and sys_platform != "cygwin"
 	pytest-home >= 0.5
+	# No Python 3.12 dependencies require importlib_metadata, but needed for type-checking since we import it directly
+	importlib_metadata; python_version >= "3.12"
 
 testing-integration =
 	pytest

setuptools/command/build_ext.py

@@ -16,7 +16,7 @@
 
 try:
     # Attempt to use Cython for building extensions, if available
-    from Cython.Distutils.build_ext import build_ext as _build_ext
+    from Cython.Distutils.build_ext import build_ext as _build_ext  # type: ignore[import-not-found] # Cython not installed on CI tests
 
     # Additionally, assert that the compiler module will load
     # also. Ref #1229.

setuptools/command/editable_wheel.py

@@ -59,7 +59,7 @@
 from .install_scripts import install_scripts as install_scripts_cls
 
 if TYPE_CHECKING:
-    from wheel.wheelfile import WheelFile  # noqa
+    from wheel.wheelfile import WheelFile  # type:ignore[import-untyped] # noqa
 
 _P = TypeVar("_P", bound=StrPath)
 _logger = logging.getLogger(__name__)

setuptools/config/_validate_pyproject/fastjsonschema_validations.py

@@ -1,5 +1,4 @@
 # noqa
-# type: ignore
 # flake8: noqa
 # pylint: skip-file
 # mypy: ignore-errors