Canonicalize name in check_if_exists

The previous implementation uses pkg_resources.get_distribution(), which
does not canonicalize the package name correctly, and fails when
combined with pip's own get_distribution(), which does canonicalize
names. This makes InstallRequirement.check_if_exists() only use pip's
own canonicalization logic so different package name forms are matched
as expected.

news/8645.bugfix

@@ -0,0 +1,2 @@
+Correctly find already-installed distributions with dot (``.``) in the name
+and uninstall them when needed.

src/pip/_internal/req/req_install.py

@@ -429,25 +429,13 @@ class InstallRequirement(object):
         """
         if self.req is None:
             return
-        # get_distribution() will resolve the entire list of requirements
-        # anyway, and we've already determined that we need the requirement
-        # in question, so strip the marker so that we don't try to
-        # evaluate it.
-        no_marker = Requirement(str(self.req))
-        no_marker.marker = None
-
-        # pkg_resources uses the canonical name to look up packages, but
-        # the name passed passed to get_distribution is not canonicalized
-        # so we have to explicitly convert it to a canonical name
-        no_marker.name = canonicalize_name(no_marker.name)
-        try:
-            self.satisfied_by = pkg_resources.get_distribution(str(no_marker))
-        except pkg_resources.DistributionNotFound:
+        existing_dist = get_distribution(self.req.name)
+        if not existing_dist:
             return
-        except pkg_resources.VersionConflict:
-            existing_dist = get_distribution(
-                self.req.name
-            )
+
+        existing_version = existing_dist.parsed_version
+        if not self.req.specifier.contains(existing_version, prereleases=True):
+            self.satisfied_by = None
             if use_user_site:
                 if dist_in_usersite(existing_dist):
                     self.should_reinstall = True
@@ -461,11 +449,13 @@ class InstallRequirement(object):
             else:
                 self.should_reinstall = True
         else:
-            if self.editable and self.satisfied_by:
+            if self.editable:
                 self.should_reinstall = True
                 # when installing editables, nothing pre-existing should ever
                 # satisfy
                 self.satisfied_by = None
+            else:
+                self.satisfied_by = existing_dist
 
     # Things valid for wheels
     @property

tests/functional/test_install_upgrade.py

@@ -1,3 +1,4 @@
+import itertools
 import os
 import sys
 import textwrap
@@ -7,6 +8,7 @@ import pytest
 from tests.lib import pyversion  # noqa: F401
 from tests.lib import assert_all_changes
 from tests.lib.local_repos import local_checkout
+from tests.lib.wheel import make_wheel
 
 
 @pytest.mark.network
@@ -439,3 +441,34 @@ class TestUpgradeDistributeToSetuptools(object):
             cwd=pip_src,
             expect_stderr=True,
         )
+
+
+@pytest.mark.parametrize("req1, req2", list(itertools.product(
+    ["foo.bar", "foo_bar", "foo-bar"], ["foo.bar", "foo_bar", "foo-bar"],
+)))
+def test_install_find_existing_package_canonicalize(script, req1, req2):
+    """Ensure an already-installed dist is found no matter how the dist name
+    was normalized on installation. (pypa/pip#8645)
+    """
+    # Create and install a package that's not available in the later stage.
+    req_container = script.scratch_path.joinpath("foo-bar")
+    req_container.mkdir()
+    req_path = make_wheel("foo_bar", "1.0").save_to_dir(req_container)
+    script.pip("install", "--no-index", req_path)
+
+    # Depend on the previously installed, but now unavailable package.
+    pkg_container = script.scratch_path.joinpath("pkg")
+    pkg_container.mkdir()
+    make_wheel(
+        "pkg",
+        "1.0",
+        metadata_updates={"Requires-Dist": req2},
+    ).save_to_dir(pkg_container)
+
+    # Ensure the previously installed package can be correctly used to match
+    # the dependency.
+    result = script.pip(
+        "install", "--no-index", "--find-links", pkg_container, "pkg",
+    )
+    satisfied_message = "Requirement already satisfied: {}".format(req2)
+    assert satisfied_message in result.stdout, str(result)