Add a --python option

docs/html/topics/index.md

@@ -18,4 +18,5 @@ local-project-installs
 repeatable-installs
 secure-installs
 vcs-support
+python-option
 ```

docs/html/topics/python-option.md

@@ -0,0 +1,29 @@
+# Managing a different Python interpreter
+
+```{versionadded} 22.3
+```
+
+Occasionally, you may want to use pip to manage a Python installation other than
+the one pip is installed into. In this case, you can use the `--python` option
+to specify the interpreter you want to manage. This option can take one of two
+values:
+
+1. The path to a Python executable.
+2. The path to a virtual environment.
+
+In both cases, pip will run exactly as if it had been invoked from that Python
+environment.
+
+One example of where this might be useful is to manage a virtual environment
+that does not have pip installed.
+
+```{pip-cli}
+$ python -m venv .venv --without-pip
+$ pip --python .venv install SomePackage
+[...]
+Successfully installed SomePackage
+```
+
+You could also use `--python .venv/bin/python` (or on Windows,
+`--python .venv\Scripts\python.exe`) if you wanted to be explicit, but the
+virtual environment name is shorter and works exactly the same.

news/11320.feature.rst

@@ -0,0 +1,2 @@
+Add a ``--python`` option to allow pip to manage Python environments other
+than the one pip is installed in.

src/pip/_internal/build_env.py

@@ -39,7 +39,7 @@ def __init__(self, path: str) -> None:
         self.lib_dirs = get_prefixed_libs(path)
 
 
-def _get_runnable_pip() -> str:
+def get_runnable_pip() -> str:
     """Get a file to pass to a Python executable, to run the currently-running pip.
 
     This is used to run a pip subprocess, for installing requirements into the build
@@ -194,7 +194,7 @@ def install_requirements(
         if not requirements:
             return
         self._install_requirements(
-            _get_runnable_pip(),
+            get_runnable_pip(),
             finder,
             requirements,
             prefix,

src/pip/_internal/cli/cmdoptions.py

@@ -189,6 +189,13 @@ class PipOption(Option):
     ),
 )
 
+python: Callable[..., Option] = partial(
+    Option,
+    "--python",
+    dest="python",
+    help="Run pip with the specified Python interpreter.",
+)
+
 verbose: Callable[..., Option] = partial(
     Option,
     "-v",
@@ -1029,6 +1036,7 @@ def check_list_path_option(options: Values) -> None:
         debug_mode,
         isolated_mode,
         require_virtualenv,
+        python,
         verbose,
         version,
         quiet,

src/pip/_internal/cli/main_parser.py

@@ -2,9 +2,11 @@
 """
 
 import os
+import subprocess
 import sys
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 
+from pip._internal.build_env import get_runnable_pip
 from pip._internal.cli import cmdoptions
 from pip._internal.cli.parser import ConfigOptionParser, UpdatingDefaultsHelpFormatter
 from pip._internal.commands import commands_dict, get_similar_commands
@@ -45,6 +47,25 @@ def create_main_parser() -> ConfigOptionParser:
     return parser
 
 
+def identify_python_interpreter(python: str) -> Optional[str]:
+    # If the named file exists, use it.
+    # If it's a directory, assume it's a virtual environment and
+    # look for the environment's Python executable.
+    if os.path.exists(python):
+        if os.path.isdir(python):
+            # bin/python for Unix, Scripts/python.exe for Windows
+            # Try both in case of odd cases like cygwin.
+            for exe in ("bin/python", "Scripts/python.exe"):
+                py = os.path.join(python, exe)
+                if os.path.exists(py):
+                    return py
+        else:
+            return python
+
+    # Could not find the interpreter specified
+    return None
+
+
 def parse_command(args: List[str]) -> Tuple[str, List[str]]:
     parser = create_main_parser()
 
@@ -57,6 +78,32 @@ def parse_command(args: List[str]) -> Tuple[str, List[str]]:
     #  args_else: ['install', '--user', 'INITools']
     general_options, args_else = parser.parse_args(args)
 
+    # --python
+    if general_options.python and "_PIP_RUNNING_IN_SUBPROCESS" not in os.environ:
+        # Re-invoke pip using the specified Python interpreter
+        interpreter = identify_python_interpreter(general_options.python)
+        if interpreter is None:
+            raise CommandError(
+                f"Could not locate Python interpreter {general_options.python}"
+            )
+
+        pip_cmd = [
+            interpreter,
+            get_runnable_pip(),
+        ]
+        pip_cmd.extend(args)
+
+        # Set a flag so the child doesn't re-invoke itself, causing
+        # an infinite loop.
+        os.environ["_PIP_RUNNING_IN_SUBPROCESS"] = "1"
+        returncode = 0
+        try:
+            proc = subprocess.run(pip_cmd)
+            returncode = proc.returncode
+        except (subprocess.SubprocessError, OSError) as exc:
+            raise CommandError(f"Failed to run pip under {interpreter}: {exc}")
+        sys.exit(returncode)
+
     # --version
     if general_options.version:
         sys.stdout.write(parser.version)

tests/functional/test_python_option.py

@@ -0,0 +1,41 @@
+import json
+import os
+from pathlib import Path
+from venv import EnvBuilder
+
+from tests.lib import PipTestEnvironment, TestData
+
+
+def test_python_interpreter(
+    script: PipTestEnvironment,
+    tmpdir: Path,
+    shared_data: TestData,
+) -> None:
+    env_path = os.fspath(tmpdir / "venv")
+    env = EnvBuilder(with_pip=False)
+    env.create(env_path)
+
+    result = script.pip("--python", env_path, "list", "--format=json")
+    before = json.loads(result.stdout)
+
+    # Ideally we would assert that before==[], but there's a problem in CI
+    # that means this isn't true. See https://github.com/pypa/pip/pull/11326
+    # for details.
+
+    script.pip(
+        "--python",
+        env_path,
+        "install",
+        "-f",
+        shared_data.find_links,
+        "--no-index",
+        "simplewheel==1.0",
+    )
+
+    result = script.pip("--python", env_path, "list", "--format=json")
+    installed = json.loads(result.stdout)
+    assert {"name": "simplewheel", "version": "1.0"} in installed
+
+    script.pip("--python", env_path, "uninstall", "simplewheel", "--yes")
+    result = script.pip("--python", env_path, "list", "--format=json")
+    assert json.loads(result.stdout) == before

tests/unit/test_cmdoptions.py

@@ -1,8 +1,12 @@
+import os
+from pathlib import Path
 from typing import Optional, Tuple
+from venv import EnvBuilder
 
 import pytest
 
 from pip._internal.cli.cmdoptions import _convert_python_version
+from pip._internal.cli.main_parser import identify_python_interpreter
 
 
 @pytest.mark.parametrize(
@@ -29,3 +33,20 @@ def test_convert_python_version(
 ) -> None:
     actual = _convert_python_version(value)
     assert actual == expected, f"actual: {actual!r}"
+
+
+def test_identify_python_interpreter_venv(tmpdir: Path) -> None:
+    env_path = tmpdir / "venv"
+    env = EnvBuilder(with_pip=False)
+    env.create(env_path)
+
+    # Passing a virtual environment returns the Python executable
+    interp = identify_python_interpreter(os.fspath(env_path))
+    assert interp is not None
+    assert Path(interp).exists()
+
+    # Passing an executable returns it
+    assert identify_python_interpreter(interp) == interp
+
+    # Passing a non-existent file returns None
+    assert identify_python_interpreter(str(tmpdir / "nonexistent")) is None