Revert 5cc540b

src/pip/_internal/metadata/pkg_resources.py

@@ -11,7 +11,6 @@
     Mapping,
     NamedTuple,
     Optional,
-    cast,
 )
 
 from pip._vendor import pkg_resources
@@ -84,19 +83,9 @@ def run_script(self, script_name: str, namespace: str) -> None:
 class Distribution(BaseDistribution):
     def __init__(self, dist: pkg_resources.Distribution) -> None:
         self._dist = dist
-        # This is populated lazily, to avoid loading metadata for all possible
-        # distributions eagerly.
-        self.__extra_mapping: Optional[Mapping[NormalizedName, str]] = None
-
-    @property
-    def _extra_mapping(self) -> Mapping[NormalizedName, str]:
-        if self.__extra_mapping is None:
-            self.__extra_mapping = {
-                canonicalize_name(extra): pkg_resources.safe_extra(cast(str, extra))
-                for extra in self.metadata.get_all("Provides-Extra", [])
-            }
-
-        return self.__extra_mapping
+        self._extra_mapping = {
+            canonicalize_name(extra): extra for extra in self._dist.extras
+        }
 
     @classmethod
     def from_directory(cls, directory: str) -> BaseDistribution:
@@ -242,10 +231,13 @@ def _metadata_impl(self) -> email.message.Message:
 
     def iter_dependencies(self, extras: Collection[str] = ()) -> Iterable[Requirement]:
         if extras:
-            relevant_extras = set(self._extra_mapping) & set(
-                map(canonicalize_name, extras)
+            sanitised_extras = {canonicalize_name(e) for e in extras} & set(
+                self._extra_mapping
             )
-            extras = [self._extra_mapping[extra] for extra in relevant_extras]
+            extras = [
+                self._extra_mapping[canonicalize_name(extra)]
+                for extra in sanitised_extras
+            ]
         return self._dist.requires(extras)
 
     def iter_provided_extras(self) -> Iterable[NormalizedName]:

tests/functional/test_install_extras.py

@@ -1,6 +1,7 @@
 import re
 import textwrap
 from os.path import join
+from pathlib import Path
 
 import pytest
 
@@ -252,3 +253,23 @@ def test_install_extras(script: PipTestEnvironment) -> None:
         "a",
     )
     script.assert_installed(a="1", b="1", dep="1", meh="1")
+
+
+def test_install_setuptools_extras_inconsistency(
+    script: PipTestEnvironment, tmp_path: Path
+) -> None:
+    test_project_path = tmp_path.joinpath("test")
+    test_project_path.mkdir()
+    test_project_path.joinpath("setup.py").write_text(
+        textwrap.dedent(
+            """
+                from setuptools import setup
+                setup(
+                    name='test',
+                    version='0.1',
+                    extras_require={'extra_underscored': ['packaging']},
+                )
+            """
+        )
+    )
+    script.pip("install", "--dry-run", test_project_path)

tests/unit/metadata/test_metadata_pkg_resources.py

@@ -40,17 +40,17 @@ def require(self, name: str) -> None:
 
 workingset = _MockWorkingSet(
     (
-        mock.Mock(test_name="global", project_name="global"),
-        mock.Mock(test_name="editable", project_name="editable"),
-        mock.Mock(test_name="normal", project_name="normal"),
-        mock.Mock(test_name="user", project_name="user"),
+        mock.Mock(test_name="global", project_name="global", extras=[]),
+        mock.Mock(test_name="editable", project_name="editable", extras=[]),
+        mock.Mock(test_name="normal", project_name="normal", extras=[]),
+        mock.Mock(test_name="user", project_name="user", extras=[]),
     )
 )
 
 workingset_stdlib = _MockWorkingSet(
     (
-        mock.Mock(test_name="normal", project_name="argparse"),
-        mock.Mock(test_name="normal", project_name="wsgiref"),
+        mock.Mock(test_name="normal", project_name="argparse", extras=[]),
+        mock.Mock(test_name="normal", project_name="wsgiref", extras=[]),
     )
 )