Separate default and ad-hoc environment APIs

pkg_resources performs annoying caching that needs to be worked around
in some parts of pip. This makes it easier to represent the difference
between environments backend by WorkingSet() and working_set.

src/pip/_internal/metadata/__init__.py

@@ -6,10 +6,27 @@
     from .base import BaseEnvironment
 
 
-def get_environment(paths=None):
+def get_default_environment():
+    # type: () -> BaseEnvironment
+    """Get the default representation for the current environment.
+
+    This returns an Environment instance from the chosen backend. The default
+    Environment instance should be built from ``sys.path`` and may use caching
+    to share instance state accorss calls.
+    """
+    from .pkg_resources import Environment
+
+    return Environment.default()
+
+
+def get_environment(paths):
     # type: (Optional[List[str]]) -> BaseEnvironment
+    """Get a representation of the environment specified by ``paths``.
+
+    This returns an Environment instance from the chosen backend based on the
+    given import paths. The backend must build a fresh instance representing
+    the state of installed distributions when this function is called.
+    """
     from .pkg_resources import Environment
 
-    if paths is None:
-        return Environment.default()
     return Environment.from_paths(paths)

src/pip/_internal/metadata/base.py

@@ -42,7 +42,7 @@ def default(cls):
 
     @classmethod
     def from_paths(cls, paths):
-        # type: (List[str]) -> BaseEnvironment
+        # type: (Optional[List[str]]) -> BaseEnvironment
         raise NotImplementedError()
 
     def get_distribution(self, name):

src/pip/_internal/metadata/pkg_resources.py

@@ -54,7 +54,7 @@ def default(cls):
 
     @classmethod
     def from_paths(cls, paths):
-        # type: (List[str]) -> BaseEnvironment
+        # type: (Optional[List[str]]) -> BaseEnvironment
         return cls(pkg_resources.WorkingSet(paths))
 
     def _search_distribution(self, name):

src/pip/_internal/self_outdated_check.py

@@ -10,7 +10,7 @@
 
 from pip._internal.index.collector import LinkCollector
 from pip._internal.index.package_finder import PackageFinder
-from pip._internal.metadata import get_environment
+from pip._internal.metadata import get_default_environment
 from pip._internal.models.selection_prefs import SelectionPreferences
 from pip._internal.utils.filesystem import adjacent_tmp_file, check_path_owner, replace
 from pip._internal.utils.misc import ensure_dir, get_installed_version
@@ -103,7 +103,7 @@ def was_installed_by_pip(pkg):
     This is used not to display the upgrade message when pip is in fact
     installed by system package manager, such as dnf on Fedora.
     """
-    dist = get_environment().get_distribution(pkg)
+    dist = get_default_environment().get_distribution(pkg)
     return dist is not None and "pip" == dist.installer
 
 

src/pip/_internal/utils/misc.py

@@ -405,9 +405,14 @@ def get_installed_distributions(
 
     Left for compatibility until direct pkg_resources uses are refactored out.
     """
-    from pip._internal.metadata import get_environment
+    from pip._internal.metadata import get_default_environment, get_environment
     from pip._internal.metadata.pkg_resources import Distribution as _Dist
-    dists = get_environment(paths).iter_installed_distributions(
+
+    if paths is None:
+        env = get_default_environment()
+    else:
+        env = get_environment(paths)
+    dists = env.iter_installed_distributions(
         local_only=local_only,
         skip=skip,
         include_editables=include_editables,
@@ -426,9 +431,9 @@ def get_distribution(req_name):
 
     Left for compatibility until direct pkg_resources uses are refactored out.
     """
-    from pip._internal.metadata import get_environment
+    from pip._internal.metadata import get_default_environment
     from pip._internal.metadata.pkg_resources import Distribution as _Dist
-    dist = get_environment().get_distribution(req_name)
+    dist = get_default_environment().get_distribution(req_name)
     if dist is None:
         return None
     return cast(_Dist, dist)._dist

tests/unit/test_self_check_outdated.py

@@ -105,7 +105,7 @@ def test_pip_self_version_check(monkeypatch, stored_time, installed_ver,
                         pretend.call_recorder(lambda *a, **kw: None))
     monkeypatch.setattr(logger, 'debug',
                         pretend.call_recorder(lambda s, exc_info=None: None))
-    monkeypatch.setattr(self_outdated_check, 'get_environment',
+    monkeypatch.setattr(self_outdated_check, 'get_default_environment',
                         lambda: MockEnvironment(installer))
 
     fake_state = pretend.stub(