fix(env): fail on incompatible Python without venv creation

src/poetry/utils/env/env_manager.py

@@ -408,6 +408,18 @@ def create_venv(
             name = self._poetry.package.name
 
         supported_python = self._poetry.package.python_constraint
+        if create_venv is False:
+            current_python = Version.parse(
+                ".".join(str(c) for c in env.version_info[:3])
+            )
+            if not supported_python.allows(current_python):
+                raise InvalidCurrentPythonVersionError(
+                    self._poetry.package.python_versions,
+                    str(current_python),
+                    "Poetry cannot switch to a compatible Python version because "
+                    "virtualenv creation is disabled.",
+                )
+
         if not supported_python.allows(python.patch_version):
             # The currently activated or chosen Python version
             # is not compatible with the Python constraint specified

src/poetry/utils/env/python/exceptions.py

@@ -31,11 +31,13 @@ def __init__(self, expected: str, given: str | None = None) -> None:
 
 
 class InvalidCurrentPythonVersionError(PythonVersionError):
-    def __init__(self, expected: str, given: str) -> None:
+    def __init__(self, expected: str, given: str, note: str | None = None) -> None:
         message = (
             f"Current Python version ({given}) "
             f"is not allowed by the project ({expected}).\n"
             'Please change python executable via the "env use" command.'
         )
+        if note is not None:
+            message = f"{message}\n{note}"
 
         super().__init__(message)

tests/utils/env/test_env_manager.py

@@ -21,6 +21,7 @@
 from poetry.utils.env import EnvManager
 from poetry.utils.env import IncorrectEnvError
 from poetry.utils.env.env_manager import EnvsFile
+from poetry.utils.env.mock_env import MockEnv
 from poetry.utils.env.python.exceptions import InvalidCurrentPythonVersionError
 from poetry.utils.env.python.exceptions import NoCompatiblePythonVersionFoundError
 from poetry.utils.env.python.exceptions import PythonVersionNotFoundError
@@ -32,6 +33,7 @@
     from collections.abc import Iterator
     from unittest.mock import MagicMock
 
+    from _pytest.monkeypatch import MonkeyPatch
     from cleo.io.buffered_io import BufferedIO
     from pytest import LogCaptureFixture
     from pytest_mock import MockerFixture
@@ -1069,6 +1071,50 @@ def test_create_venv_fails_if_no_compatible_python_version_could_be_found(
     assert m.call_count == 0
 
 
+@pytest.mark.parametrize("use_poetry_python", [True, False])
+def test_create_venv_fails_if_current_python_is_not_supported_without_creating_venv(
+    manager: EnvManager,
+    poetry: Poetry,
+    config: Config,
+    mocker: MockerFixture,
+    mocked_python_register: MockedPythonRegister,
+    monkeypatch: MonkeyPatch,
+    use_poetry_python: bool,
+) -> None:
+    config.config["virtualenvs"]["create"] = False
+    config.config["virtualenvs"]["use-poetry-python"] = use_poetry_python
+    monkeypatch.delenv("VIRTUAL_ENV", raising=False)
+
+    poetry.package.python_versions = "^3.10"
+
+    current_python = mocked_python_register("3.9.0")
+    compatible_python = mocked_python_register("3.10.0")
+    mocker.patch.object(manager, "get", return_value=MockEnv(version_info=(3, 9, 0)))
+    mocker.patch(
+        "poetry.utils.env.env_manager.Python.get_preferred_python",
+        return_value=current_python if use_poetry_python else compatible_python,
+    )
+    get_compatible_python = mocker.patch(
+        "poetry.utils.env.env_manager.Python.get_compatible_python",
+        return_value=compatible_python,
+    )
+    build_venv = mocker.patch("poetry.utils.env.EnvManager.build_venv")
+
+    with pytest.raises(InvalidCurrentPythonVersionError) as e:
+        manager.create_venv()
+
+    expected_message = (
+        "Current Python version (3.9.0) is not allowed by the project (^3.10).\n"
+        'Please change python executable via the "env use" command.\n'
+        "Poetry cannot switch to a compatible Python version because virtualenv "
+        "creation is disabled."
+    )
+
+    assert str(e.value) == expected_message
+    get_compatible_python.assert_not_called()
+    build_venv.assert_not_called()
+
+
 @pytest.mark.parametrize("use_poetry_python", [True, False])
 def test_create_venv_does_not_try_to_find_compatible_versions_with_executable(
     manager: EnvManager,