gh-146310: Fix ensurepip to treat empty WHEEL_PKG_DIR as unset

Path('') resolves to CWD, so an empty WHEEL_PKG_DIR string caused
ensurepip to search the current working directory for wheel files.
Add a truthiness check to treat empty strings the same as None.

Author: kimimgo

Lib/ensurepip/__init__.py

@@ -16,7 +16,7 @@
 # policies recommend against bundling dependencies. For example, Fedora
 # installs wheel packages in the /usr/share/python-wheels/ directory and don't
 # install the ensurepip._bundled package.
-if (_pkg_dir := sysconfig.get_config_var('WHEEL_PKG_DIR')) is not None:
+if (_pkg_dir := sysconfig.get_config_var('WHEEL_PKG_DIR')) is not None and _pkg_dir:
     _WHEEL_PKG_DIR = Path(_pkg_dir).resolve()
 else:
     _WHEEL_PKG_DIR = None

Lib/test/test_ensurepip.py

@@ -36,6 +36,13 @@ def test_version_no_dir(self):
             # when the bundled pip wheel is used, we get _PIP_VERSION
             self.assertEqual(ensurepip._PIP_VERSION, ensurepip.version())
 
+    def test_empty_wheel_pkg_dir_treated_as_none(self):
+        # GH#146310: empty string WHEEL_PKG_DIR should not search CWD.
+        # An empty WHEEL_PKG_DIR converts to Path('.') which would
+        # incorrectly search the current working directory.
+        with unittest.mock.patch.object(ensurepip, '_WHEEL_PKG_DIR', None):
+            self.assertIsNone(ensurepip._find_wheel_pkg_dir_pip())
+
     def test_selected_wheel_path_no_dir(self):
         pip_filename = f'pip-{ensurepip._PIP_VERSION}-py3-none-any.whl'
         with unittest.mock.patch.object(ensurepip, '_WHEEL_PKG_DIR', None):

Misc/NEWS.d/next/Library/2026-03-24-03-49-50.gh-issue-146310.WhlDir.rst

@@ -0,0 +1,2 @@
+Fix :mod:`ensurepip` to treat an empty string ``WHEEL_PKG_DIR`` as unset,
+preventing it from searching the current working directory for wheel files.