fix(env): treat empty VIRTUAL_ENV/CONDA_PREFIX as unset

src/poetry/utils/env/env_manager.py

@@ -159,7 +159,7 @@ def activate(self, python: str) -> Env:
 
         # Create if needed
         if not venv.exists() or create:
-            in_venv = os.environ.get("VIRTUAL_ENV") is not None
+            in_venv = bool(os.environ.get("VIRTUAL_ENV"))
             if in_venv or not venv.exists():
                 create = True
 
@@ -214,7 +214,9 @@ def get(self, reload: bool = False) -> Env:
         conda_env_name = os.environ.get("CONDA_DEFAULT_ENV")
         # It's probably not a good idea to pollute Conda's global "base" env, since
         # most users have it activated all the time.
-        in_venv = env_prefix is not None and conda_env_name != "base"
+        # Treat an empty env_prefix as if no virtualenv is active, since conda
+        # can leave CONDA_PREFIX set to an empty string after deactivation.
+        in_venv = bool(env_prefix) and conda_env_name != "base"
 
         if not in_venv or env is not None:
             # Checking if a local virtualenv exists
@@ -249,14 +251,8 @@ def get(self, reload: bool = False) -> Env:
 
             return VirtualEnv(venv)
 
-        if env_prefix is not None:
-            prefix = Path(env_prefix)
-            base_prefix = None
-        else:
-            prefix = Path(sys.prefix)
-            base_prefix = self.get_base_prefix()
-
-        return VirtualEnv(prefix, base_prefix)
+        assert env_prefix
+        return VirtualEnv(Path(env_prefix))
 
     def list(self, name: str | None = None) -> list[VirtualEnv]:
         if name is None:

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

@@ -84,7 +84,7 @@ def __init__(
     @classmethod
     def find_all(cls) -> Iterator[Python]:
         venv_path: Path | None = (
-            Path(os.environ["VIRTUAL_ENV"]) if "VIRTUAL_ENV" in os.environ else None
+            Path(venv) if (venv := os.environ.get("VIRTUAL_ENV")) else None
         )
         for python in findpython.find_all():
             if venv_path and python.executable.is_relative_to(venv_path):

tests/utils/env/test_env_manager.py

@@ -577,6 +577,33 @@ def test_get_prefers_explicitly_activated_virtualenvs_over_env_var(
     assert env.base == Path(sys.base_prefix)
 
 
+@pytest.mark.parametrize("env_var", ["VIRTUAL_ENV", "CONDA_PREFIX"])
+def test_get_ignores_empty_env_prefix(
+    manager: EnvManager,
+    poetry: Poetry,
+    in_project_venv_dir: Path,
+    env_var: str,
+    mocker: MockerFixture,
+) -> None:
+    """An empty VIRTUAL_ENV or CONDA_PREFIX should be treated as unset.
+
+    After ``conda deactivate``, conda can leave CONDA_PREFIX set to an
+    empty string.  Poetry should not consider that as an active
+    virtualenv and should fall back to the in-project .venv instead.
+
+    See: https://github.com/python-poetry/poetry/issues/10770
+    """
+    os.environ.pop("VIRTUAL_ENV", None)
+    os.environ.pop("CONDA_PREFIX", None)
+    os.environ[env_var] = ""
+    mocker.patch(
+        "poetry.utils.env.virtual_env.VirtualEnv.__init__",
+        lambda self, *args, **kwargs: setattr(self, "_path", args[0]),
+    )
+    venv = manager.get()
+    assert venv.path == in_project_venv_dir
+
+
 def test_list(
     tmp_path: Path,
     manager: EnvManager,