Check current python executable version (#4520)

* Check current python version

* fix current_python type

src/poetry/utils/env.py

@@ -450,6 +450,17 @@ class NoCompatiblePythonVersionFound(EnvError):
         super().__init__(message)
 
 
+class InvalidCurrentPythonVersionError(EnvError):
+    def __init__(self, expected: str, given: str) -> 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.'
+        )
+
+        super().__init__(message)
+
+
 class EnvManager:
     """
     Environments manager
@@ -809,6 +820,13 @@ class EnvManager:
 
         if env.is_venv() and not force:
             # Already inside a virtualenv.
+            current_python = Version.parse(
+                ".".join(str(c) for c in env.version_info[:3])
+            )
+            if not self._poetry.package.python_constraint.allows(current_python):
+                raise InvalidCurrentPythonVersionError(
+                    self._poetry.package.python_versions, str(current_python)
+                )
             return env
 
         create_venv = self._poetry.config.get("virtualenvs.create")

tests/utils/test_env.py

@@ -24,6 +24,7 @@ from poetry.utils.env import GET_BASE_PREFIX
 from poetry.utils.env import EnvCommandError
 from poetry.utils.env import EnvManager
 from poetry.utils.env import GenericEnv
+from poetry.utils.env import InvalidCurrentPythonVersionError
 from poetry.utils.env import NoCompatiblePythonVersionFound
 from poetry.utils.env import SystemEnv
 from poetry.utils.env import VirtualEnv
@@ -993,6 +994,33 @@ def test_create_venv_uses_patch_version_to_detect_compatibility_with_executable(
     )
 
 
+def test_create_venv_fails_if_current_python_version_is_not_supported(
+    manager: EnvManager, poetry: "Poetry"
+):
+    if "VIRTUAL_ENV" in os.environ:
+        del os.environ["VIRTUAL_ENV"]
+
+    manager.create_venv(NullIO())
+
+    current_version = Version.parse(".".join(str(c) for c in sys.version_info[:3]))
+    next_version = ".".join(
+        str(c) for c in (current_version.major, current_version.minor + 1, 0)
+    )
+    package_version = "~" + next_version
+    poetry.package.python_versions = package_version
+
+    with pytest.raises(InvalidCurrentPythonVersionError) as e:
+        manager.create_venv(NullIO())
+
+    expected_message = (
+        f"Current Python version ({current_version}) is not allowed by the project"
+        f' ({package_version}).\nPlease change python executable via the "env use"'
+        " command."
+    )
+
+    assert expected_message == str(e.value)
+
+
 def test_activate_with_in_project_setting_does_not_fail_if_no_venvs_dir(
     manager: EnvManager,
     poetry: Poetry,