Move utils.misc.cast to avoid circular import

src/pip/_internal/utils/filesystem.py

@@ -13,8 +13,7 @@ from pip._vendor.retrying import retry  # type: ignore
 from pip._vendor.six import PY2
 
 from pip._internal.utils.compat import get_path_uid
-from pip._internal.utils.misc import cast
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING, cast
-from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
     from typing import BinaryIO, Iterator

src/pip/_internal/utils/misc.py

@@ -39,7 +39,7 @@ from pip._internal.utils.compat import (
     str_to_display,
 )
 from pip._internal.utils.marker_files import write_delete_marker_file
-from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING, cast
 from pip._internal.utils.virtualenv import (
     running_under_virtualenv,
     virtualenv_no_global,
@@ -53,16 +53,11 @@ else:
 if MYPY_CHECK_RUNNING:
     from typing import (
         Any, AnyStr, Container, Iterable, List, Optional, Text,
-        Tuple, Union, cast,
+        Tuple, Union,
     )
     from pip._vendor.pkg_resources import Distribution
 
     VersionInfo = Tuple[int, int, int]
-else:
-    # typing's cast() is needed at runtime, but we don't want to import typing.
-    # Thus, we use a dummy no-op version, which we tell mypy to ignore.
-    def cast(type_, value):  # type: ignore
-        return value
 
 
 __all__ = ['rmtree', 'display_path', 'backup_dir',

src/pip/_internal/utils/typing.py

@@ -27,3 +27,12 @@ Ref: https://github.com/python/mypy/issues/3216
 """
 
 MYPY_CHECK_RUNNING = False
+
+
+if MYPY_CHECK_RUNNING:
+    from typing import cast
+else:
+    # typing's cast() is needed at runtime, but we don't want to import typing.
+    # Thus, we use a dummy no-op version, which we tell mypy to ignore.
+    def cast(type_, value):  # type: ignore
+        return value