Implement `--break-system-packages` for EXTERNALLY-MANAGED installations (#11780)

The PEP 668 expects an override mechanism to ease the transition.
This provides an override.

---------

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

6 files changed, 37 insertions, 3 deletions

diff --git a/news/11780.feature.rst b/news/11780.feature.rst
new file mode 100644
index 000000000..b765de6c5
--- /dev/null
+++ b/news/11780.feature.rst
@@ -0,0 +1,2 @@
+Implement ``--break-system-packages`` to permit installing packages into
+``EXTERNALLY-MANAGED`` Python installations.
diff --git a/src/pip/_internal/cli/cmdoptions.py b/src/pip/_internal/cli/cmdoptions.py
index 661c489c7..1f804097e 100644
--- a/src/pip/_internal/cli/cmdoptions.py
+++ b/src/pip/_internal/cli/cmdoptions.py
@@ -164,6 +164,14 @@ require_virtualenv: Callable[..., Option] = partial(
     ),
 )
 
+override_externally_managed: Callable[..., Option] = partial(
+    Option,
+    "--break-system-packages",
+    dest="override_externally_managed",
+    action="store_true",
+    help="Allow pip to modify an EXTERNALLY-MANAGED Python installation",
+)
+
 python: Callable[..., Option] = partial(
     Option,
     "--python",
diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index cecaac2bc..b20aeddf8 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -215,6 +215,7 @@ class InstallCommand(RequirementCommand):
         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 @@ class InstallCommand(RequirementCommand):
             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"
diff --git a/src/pip/_internal/commands/uninstall.py b/src/pip/_internal/commands/uninstall.py
index e5a4c8e10..f198fc313 100644
--- a/src/pip/_internal/commands/uninstall.py
+++ b/src/pip/_internal/commands/uninstall.py
@@ -58,6 +58,7 @@ class UninstallCommand(Command, SessionCommandMixin):
             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:
@@ -93,7 +94,8 @@ class UninstallCommand(Command, SessionCommandMixin):
                 f'"pip help {self.name}")'
             )
 
-        check_externally_managed()
+        if not options.override_externally_managed:
+            check_externally_managed()
 
         protect_pip_from_modification_on_windows(
             modifying_pip="pip" in reqs_to_uninstall
diff --git a/src/pip/_internal/exceptions.py b/src/pip/_internal/exceptions.py
index d28713ff7..d4527295d 100644
--- a/src/pip/_internal/exceptions.py
+++ b/src/pip/_internal/exceptions.py
@@ -696,7 +696,9 @@ class ExternallyManagedEnvironment(DiagnosticPipError):
             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."),
         )
diff --git a/tests/functional/test_pep668.py b/tests/functional/test_pep668.py
index 1fed85e70..3c1085668 100644
--- a/tests/functional/test_pep668.py
+++ b/tests/functional/test_pep668.py
@@ -45,6 +45,22 @@ def test_fails(script: PipTestEnvironment, arguments: List[str]) -> None:
 @pytest.mark.parametrize(
     "arguments",
     [
+        pytest.param(["install"], id="install"),
+        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", "--break-system-packages")
+    assert "I am externally managed" not in result.stderr
+
+
+@pytest.mark.parametrize(
+    "arguments",
+    [
         pytest.param(["install", "--root"], id="install-root"),
         pytest.param(["install", "--prefix"], id="install-prefix"),
         pytest.param(["install", "--target"], id="install-target"),