Try to detect if we are in a virtual environment and change path precedence accordingly

jupyter_core/command.py

@@ -231,13 +231,13 @@ def main():
                 if args.debug:
                     env = os.environ
 
-                    if paths.envset("JUPYTER_PREFER_ENV_PATH"):
+                    if paths.prefer_environment_over_user():
                         print(
-                            "JUPYTER_PREFER_ENV_PATH is set, making the environment-level path preferred over the user-level path for data and config"
+                            "JUPYTER_PREFER_ENV_PATH is set to a true value, or JUPYTER_PREFER_ENV_PATH is not set and we detected a virtual environment, making the environment-level path preferred over the user-level path for data and config"
                         )
                     else:
                         print(
-                            "JUPYTER_PREFER_ENV_PATH is not set, making the user-level path preferred over the environment-level path for data and config"
+                            "JUPYTER_PREFER_ENV_PATH is set to a false value, or JUPYTER_PREFER_ENV_PATH is not set and we did not detect a virtual environment, making the user-level path preferred over the environment-level path for data and config"
                         )
 
                     # config path list

jupyter_core/paths.py

@@ -26,13 +26,18 @@
 UF_HIDDEN = getattr(stat, "UF_HIDDEN", 32768)
 
 
-def envset(name):
-    """Return True if the given environment variable is set
+def envset(name, default=False):
+    """Return the boolean value of a given environment variable.
 
     An environment variable is considered set if it is assigned to a value
     other than 'no', 'n', 'false', 'off', '0', or '0.0' (case insensitive)
+
+    If the environment variable is not defined, the default value is returned.
     """
-    return os.environ.get(name, "no").lower() not in ["no", "n", "false", "off", "0", "0.0"]
+    if name not in os.environ:
+        return default
+
+    return os.environ[name].lower() not in ["no", "n", "false", "off", "0", "0.0"]
 
 
 def get_home_dir():
@@ -47,6 +52,23 @@ def get_home_dir():
 _dtemps: dict = {}
 
 
+def prefer_environment_over_user():
+    """Determine if environment-level paths should take precedence over user-level paths."""
+    # If JUPYTER_PREFER_ENV_PATH is defined, that signals user intent, so return its value
+    if "JUPYTER_PREFER_ENV_PATH" in os.environ:
+        return envset("JUPYTER_PREFER_ENV_PATH")
+
+    # If we are in a Python virtualenv, default to True (see https://docs.python.org/3/library/venv.html#venv-def)
+    if sys.prefix != sys.base_prefix:
+        return True
+
+    # If sys.prefix indicates Python comes from a conda/mamba environment, default to True
+    if "CONDA_PREFIX" in os.environ and sys.prefix.startswith(os.environ["CONDA_PREFIX"]):
+        return True
+
+    return False
+
+
 def _mkdtemp_once(name):
     """Make or reuse a temporary directory.
 
@@ -184,7 +206,7 @@ def jupyter_path(*subdirs):
 
     env = [p for p in ENV_JUPYTER_PATH if p not in SYSTEM_JUPYTER_PATH]
 
-    if envset("JUPYTER_PREFER_ENV_PATH"):
+    if prefer_environment_over_user():
         paths.extend(env)
         paths.extend(user)
     else:
@@ -253,7 +275,7 @@ def jupyter_config_path():
 
     env = [p for p in ENV_CONFIG_PATH if p not in SYSTEM_CONFIG_PATH]
 
-    if envset("JUPYTER_PREFER_ENV_PATH"):
+    if prefer_environment_over_user():
         paths.extend(env)
         paths.extend(user)
     else:

jupyter_core/tests/test_command.py

@@ -23,7 +23,6 @@
 
 def setup_module():
     resetenv.start()
-    os.environ.pop("JUPYTER_PREFER_ENV_PATH", None)
 
 
 def teardown_module():

jupyter_core/tests/test_paths.py

@@ -25,6 +25,7 @@
     jupyter_data_dir,
     jupyter_path,
     jupyter_runtime_dir,
+    prefer_environment_over_user,
     secure_write,
 )
 
@@ -61,13 +62,16 @@
 jupyter_config_env = "/jupyter-cfg"
 config_env = patch.dict("os.environ", {"JUPYTER_CONFIG_DIR": jupyter_config_env})
 prefer_env = patch.dict("os.environ", {"JUPYTER_PREFER_ENV_PATH": "True"})
+prefer_user = patch.dict("os.environ", {"JUPYTER_PREFER_ENV_PATH": "False"})
 
 resetenv = patch.dict(os.environ)
 
 
 def setup_module():
     resetenv.start()
-    os.environ.pop("JUPYTER_PREFER_ENV_PATH", None)
+    # For these tests, default to preferring the user-level over environment-level paths
+    # Tests can override this preference using the prefer_env decorator/context manager
+    os.environ["JUPYTER_PREFER_ENV_PATH"] = "no"
 
 
 def teardown_module():
@@ -89,7 +93,10 @@ def test_envset():
             assert paths.envset(f"FOO_{v}")
         for v in false_values:
             assert not paths.envset(f"FOO_{v}")
-        assert not paths.envset("THIS_VARIABLE_SHOULD_NOT_BE_SET")
+        # Test default value is False
+        assert paths.envset("THIS_VARIABLE_SHOULD_NOT_BE_SET") is False
+        # Test envset returns the given default if supplied
+        assert paths.envset("THIS_VARIABLE_SHOULD_NOT_BE_SET", None) is None
 
 
 @pytest.mark.skipif(sys.platform == "win32", reason="does not run on windows")
@@ -328,6 +335,21 @@ def test_jupyter_config_path_env():
     assert path[:2] == [pjoin("foo", "bar"), pjoin("bar", "baz")]
 
 
+def test_prefer_environment_over_user():
+    with prefer_env:
+        assert prefer_environment_over_user() is True
+
+    with prefer_user:
+        assert prefer_environment_over_user() is False
+
+    # Test default if environment variable is not set, and try to determine if we are in a virtual environment
+    os.environ.pop("JUPYTER_PREFER_ENV_PATH", None)
+    in_venv = sys.prefix != sys.base_prefix or (
+        "CONDA_PREFIX" in os.environ and sys.prefix.startswith(os.environ["CONDA_PREFIX"])
+    )
+    assert prefer_environment_over_user() is in_venv
+
+
 def test_is_hidden():
     with tempfile.TemporaryDirectory() as root:
         subdir1 = os.path.join(root, "subdir")