Implement pip install --break-system-packages, as a PEP-668 override

The PEP expected an override mechanism to ease the transition to PEP-668
managed installs. This provides a command-line-driven override.

news/11780.feature.rst

@@ -0,0 +1,2 @@
+Implement ``--break-system-packages`` to permit installing packages into
+``EXTERNALLY-MANAGED`` Python installations.

src/pip/_internal/cli/cmdoptions.py

@@ -164,6 +164,14 @@ class PipOption(Option):
     ),
 )
 
+override_externally_managed: Callable[..., Option] = partial(
+    Option,
+    "--break-system-packages",
+    dest="override_externally_managed",
+    action="store_true",
+    help="Allow pip to install into an EXTERNALLY-MANAGED Python installation",
+)
+
 python: Callable[..., Option] = partial(
     Option,
     "--python",

src/pip/_internal/commands/install.py

@@ -215,6 +215,7 @@ def add_options(self) -> None:
         self.cmd_opts.add_option(cmdoptions.use_pep517())
         self.cmd_opts.add_option(cmdoptions.no_use_pep517())
         self.cmd_opts.add_option(cmdoptions.check_build_deps())
+        self.cmd_opts.add_option(cmdoptions.override_externally_managed())
 
         self.cmd_opts.add_option(cmdoptions.config_settings())
         self.cmd_opts.add_option(cmdoptions.install_options())
@@ -296,7 +297,10 @@ def run(self, options: Values, args: List[str]) -> int:
             and options.target_dir is None
             and options.prefix_path is None
         )
-        if installing_into_current_environment:
+        if (
+            installing_into_current_environment
+            and not options.override_externally_managed
+        ):
             check_externally_managed()
 
         upgrade_strategy = "to-satisfy-only"

src/pip/_internal/commands/uninstall.py

@@ -58,6 +58,7 @@ def add_options(self) -> None:
             help="Don't ask for confirmation of uninstall deletions.",
         )
         self.cmd_opts.add_option(cmdoptions.root_user_action())
+        self.cmd_opts.add_option(cmdoptions.override_externally_managed())
         self.parser.insert_option_group(0, self.cmd_opts)
 
     def run(self, options: Values, args: List[str]) -> int:

src/pip/_internal/exceptions.py

@@ -696,7 +696,9 @@ def __init__(self, error: Optional[str]) -> None:
             context=context,
             note_stmt=(
                 "If you believe this is a mistake, please contact your "
-                "Python installation or OS distribution provider."
+                "Python installation or OS distribution provider. "
+                "You can override this, at the risk of breaking your Python "
+                "installation or OS, by passing --break-system-packages."
             ),
             hint_stmt=Text("See PEP 668 for the detailed specification."),
         )

tests/functional/test_pep668.py

@@ -42,6 +42,23 @@ def test_fails(script: PipTestEnvironment, arguments: List[str]) -> None:
     assert "I am externally managed" in result.stderr
 
 
+@pytest.mark.parametrize(
+    "arguments",
+    [
+        pytest.param(["install"], id="install"),
+        pytest.param(["install", "--user"], id="install-user"),
+        pytest.param(["install", "--dry-run"], id="install-dry-run"),
+        pytest.param(["uninstall", "-y"], id="uninstall"),
+    ],
+)
+@pytest.mark.usefixtures("patch_check_externally_managed")
+def test_succeeds_when_overridden(
+    script: PipTestEnvironment, arguments: List[str]
+) -> None:
+    result = script.pip(*arguments, "pip")
+    assert "I am externally managed" not in result.stderr
+
+
 @pytest.mark.parametrize(
     "arguments",
     [