Use explicit default value for TempDirectory delete flag

Now we can opt-in to globally-managed + globally-configured file
deletion for pre-existing directories by passing an explicit `None`.

Author: chrahunt

src/pip/_internal/utils/temp_dir.py

@@ -13,7 +13,7 @@
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
-    from typing import Any, Dict, Iterator, Optional, TypeVar
+    from typing import Any, Dict, Iterator, Optional, TypeVar, Union
 
     _T = TypeVar('_T', bound='TempDirectory')
 
@@ -77,6 +77,13 @@ def tempdir_registry():
         _tempdir_registry = old_tempdir_registry
 
 
+class _Default(object):
+    pass
+
+
+_default = _Default()
+
+
 class TempDirectory(object):
     """Helper class that owns and cleans up a temporary directory.
 
@@ -101,16 +108,21 @@ class TempDirectory(object):
     def __init__(
         self,
         path=None,    # type: Optional[str]
-        delete=None,  # type: Optional[bool]
+        delete=_default,  # type: Union[bool, None, _Default]
         kind="temp",  # type: str
         globally_managed=False,  # type: bool
     ):
         super(TempDirectory, self).__init__()
 
-        # If we were given an explicit directory, resolve delete option now.
-        # Otherwise we wait until cleanup and see what tempdir_registry says.
-        if path is not None and delete is None:
-            delete = False
+        if delete is _default:
+            if path is not None:
+                # If we were given an explicit directory, resolve delete option
+                # now.
+                delete = False
+            else:
+                # Otherwise, we wait until cleanup and see what
+                # tempdir_registry says.
+                delete = None
 
         if path is None:
             path = self._create(kind)

tests/unit/test_utils_temp_dir.py

@@ -10,6 +10,7 @@
 from pip._internal.utils.temp_dir import (
     AdjacentTempDirectory,
     TempDirectory,
+    _default,
     global_tempdir_manager,
     tempdir_registry,
 )
@@ -216,12 +217,15 @@ def test_tempdirectory_asserts_global_tempdir(monkeypatch):
 
 @pytest.mark.parametrize("delete,kind,exists", [
     (None, deleted_kind, False),
+    (_default, deleted_kind, False),
     (True, deleted_kind, False),
     (False, deleted_kind, True),
     (None, not_deleted_kind, True),
+    (_default, not_deleted_kind, True),
     (True, not_deleted_kind, False),
     (False, not_deleted_kind, True),
     (None, "unspecified", False),
+    (_default, "unspecified", False),
     (True, "unspecified", False),
     (False, "unspecified", True),
 ])
@@ -236,6 +240,24 @@ def test_tempdir_registry(kind, delete, exists):
         assert os.path.exists(path) == exists
 
 
+@pytest.mark.parametrize("delete,exists", [
+    (_default, True), (None, False)
+])
+def test_temp_dir_does_not_delete_explicit_paths_by_default(
+    tmpdir, delete, exists
+):
+    path = tmpdir / "example"
+    path.mkdir()
+
+    with tempdir_registry() as registry:
+        registry.set_delete(deleted_kind, True)
+
+        with TempDirectory(path=path, delete=delete, kind=deleted_kind) as d:
+            assert str(d.path) == path
+            assert os.path.exists(path)
+        assert os.path.exists(path) == exists
+
+
 @pytest.mark.parametrize("should_delete", [True, False])
 def test_tempdir_registry_lazy(should_delete):
     """