Issue #19728: add private ensurepip._uninstall CLI

MvL would like to be able to preserve CPython's existing clean
uninstall behaviour on Windows before enabling the pip
installation option by default.

This private CLI means running "python -m ensurepip._uninstall"
will remove pip and setuptools before proceeding with the rest
of the uninstallation process.

If the version of pip differs from the one bootstrapped by
CPython, then the uninstallation helper will leave it alone
(just like any other pip installed packages)

Lib/test/test_ensurepip.py

@@ -4,6 +4,8 @@ import ensurepip
 import test.support
 import os
 import os.path
+import contextlib
+import sys
 
 
 class TestEnsurePipVersion(unittest.TestCase):
@@ -122,6 +124,79 @@ class TestBootstrap(unittest.TestCase):
     def test_altinstall_default_pip_conflict(self):
         with self.assertRaises(ValueError):
             ensurepip.bootstrap(altinstall=True, default_pip=True)
+        self.run_pip.assert_not_called()
+
+@contextlib.contextmanager
+def fake_pip(version=ensurepip._PIP_VERSION):
+    if version is None:
+        pip = None
+    else:
+        class FakePip():
+            __version__ = version
+        pip = FakePip()
+    sentinel = object()
+    orig_pip = sys.modules.get("pip", sentinel)
+    sys.modules["pip"] = pip
+    try:
+        yield pip
+    finally:
+        if orig_pip is sentinel:
+            del sys.modules["pip"]
+        else:
+            sys.modules["pip"] = orig_pip
+
+class TestUninstall(unittest.TestCase):
+
+    def setUp(self):
+        run_pip_patch = unittest.mock.patch("ensurepip._run_pip")
+        self.run_pip = run_pip_patch.start()
+        self.addCleanup(run_pip_patch.stop)
+
+    def test_uninstall_skipped_when_not_installed(self):
+        with fake_pip(None):
+            ensurepip._uninstall()
+        self.run_pip.assert_not_called()
+
+    def test_uninstall_fails_with_wrong_version(self):
+        with fake_pip("not a valid version"):
+            with self.assertRaises(RuntimeError):
+                ensurepip._uninstall()
+        self.run_pip.assert_not_called()
+
+
+    def test_uninstall(self):
+        with fake_pip():
+            ensurepip._uninstall()
+
+        self.run_pip.assert_called_once_with(
+            ["uninstall", "-y", "pip", "setuptools"]
+        )
+
+    def test_uninstall_with_verbosity_1(self):
+        with fake_pip():
+            ensurepip._uninstall(verbosity=1)
+
+        self.run_pip.assert_called_once_with(
+            ["uninstall", "-y", "-v", "pip", "setuptools"]
+        )
+
+    def test_uninstall_with_verbosity_2(self):
+        with fake_pip():
+            ensurepip._uninstall(verbosity=2)
+
+        self.run_pip.assert_called_once_with(
+            ["uninstall", "-y", "-vv", "pip", "setuptools"]
+        )
+
+    def test_uninstall_with_verbosity_3(self):
+        with fake_pip():
+            ensurepip._uninstall(verbosity=3)
+
+        self.run_pip.assert_called_once_with(
+            ["uninstall", "-y", "-vvv", "pip", "setuptools"]
+        )
+
+
 
 
 if __name__ == "__main__":