Use existing .venv even without explicit in-project

src/poetry/utils/env.py

@@ -621,7 +621,7 @@ def activate(self, python: str) -> Env:
         patch = python_version.text
 
         create = False
-        is_root_venv = self._poetry.config.get("virtualenvs.in-project")
+        is_root_venv = self.use_root_env()
         # If we are required to create the virtual environment in the root folder,
         # create or recreate it if needed
         if is_root_venv:
@@ -730,11 +730,7 @@ def get(self, reload: bool = False) -> Env:
 
         if not in_venv or env is not None:
             # Checking if a local virtualenv exists
-            if (
-                self._poetry.config.get("virtualenvs.in-project") is not False
-                and (cwd / ".venv").exists()
-                and (cwd / ".venv").is_dir()
-            ):
+            if self.use_root_env():
                 venv = cwd / ".venv"
 
                 return VirtualEnv(venv)
@@ -773,11 +769,7 @@ def list(self, name: str | None = None) -> list[VirtualEnv]:
         env_list = [VirtualEnv(p) for p in sorted(venv_path.glob(f"{venv_name}-py*"))]
 
         venv = self._poetry.file.parent / ".venv"
-        if (
-            self._poetry.config.get("virtualenvs.in-project") is not False
-            and venv.exists()
-            and venv.is_dir()
-        ):
+        if self.use_root_env():
             env_list.insert(0, VirtualEnv(venv))
         return env_list
 
@@ -891,6 +883,14 @@ def remove(self, python: str) -> Env:
 
         return VirtualEnv(venv_path, venv_path)
 
+    def use_root_env(self) -> bool:
+        in_project: bool | None = self._poetry.config.get("virtualenvs.in-project")
+        if in_project is not None:
+            return in_project
+
+        root_venv: Path = self._poetry.file.parent / ".venv"
+        return root_venv.exists() and root_venv.is_dir()
+
     def create_venv(
         self,
         name: str | None = None,
@@ -918,7 +918,7 @@ def create_venv(
             return env
 
         create_venv = self._poetry.config.get("virtualenvs.create")
-        root_venv = self._poetry.config.get("virtualenvs.in-project")
+        root_venv = self.use_root_env()
         prefer_active_python = self._poetry.config.get(
             "virtualenvs.prefer-active-python"
         )

tests/utils/test_env.py

@@ -208,6 +208,52 @@ def check_output(cmd: list[str], *args: Any, **kwargs: Any) -> str:
     return check_output
 
 
+def test_activate_in_project_venv_no_explicit_config(
+    tmp_path: Path,
+    manager: EnvManager,
+    poetry: Poetry,
+    mocker: MockerFixture,
+    venv_name: str,
+    in_project_venv_dir: Path,
+) -> None:
+    mocker.patch("shutil.which", side_effect=lambda py: f"/usr/bin/{py}")
+    mocker.patch(
+        "subprocess.check_output",
+        side_effect=check_output_wrapper(),
+    )
+    mocker.patch(
+        "subprocess.Popen.communicate",
+        side_effect=[
+            ("/prefix", None),
+            ('{"version_info": [3, 7, 0]}', None),
+            ("/prefix", None),
+            ("/prefix", None),
+            ("/prefix", None),
+        ],
+    )
+    m = mocker.patch("poetry.utils.env.EnvManager.build_venv", side_effect=build_venv)
+
+    env = manager.activate("python3.7")
+
+    assert env.path == tmp_path / "poetry-fixture-simple" / ".venv"
+    assert env.base == Path("/prefix")
+
+    m.assert_called_with(
+        tmp_path / "poetry-fixture-simple" / ".venv",
+        executable=Path("/usr/bin/python3.7"),
+        flags={
+            "always-copy": False,
+            "system-site-packages": False,
+            "no-pip": False,
+            "no-setuptools": False,
+        },
+        prompt="simple-project-py3.7",
+    )
+
+    envs_file = TOMLFile(tmp_path / "envs.toml")
+    assert not envs_file.exists()
+
+
 def test_activate_activates_non_existing_virtualenv_no_envs_file(
     tmp_path: Path,
     manager: EnvManager,
@@ -1378,7 +1424,7 @@ def test_activate_with_in_project_setting_does_not_fail_if_no_venvs_dir(
     )
     mocker.patch(
         "subprocess.Popen.communicate",
-        side_effect=[("/prefix", None), ("/prefix", None)],
+        side_effect=[("/prefix", None), ("/prefix", None), ("/prefix", None)],
     )
     m = mocker.patch("poetry.utils.env.EnvManager.build_venv")