1 files changed, 29 insertions, 0 deletions
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 86dad4a3a..3f7de0061 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -5,6 +5,7 @@ import sys
 import sysconfig
 import typing
 
+from pip._internal.exceptions import InstallationError
 from pip._internal.utils import appdirs
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
@@ -23,6 +24,34 @@ def get_major_minor_version() -> str:
     return "{}.{}".format(*sys.version_info)
 
 
+def change_root(new_root: str, pathname: str) -> str:
+    """Return 'pathname' with 'new_root' prepended.
+
+    If 'pathname' is relative, this is equivalent to os.path.join(new_root, pathname).
+    Otherwise, it requires making 'pathname' relative and then joining the
+    two, which is tricky on DOS/Windows and Mac OS.
+
+    This is borrowed from Python's standard library's distutils module.
+    """
+    if os.name == "posix":
+        if not os.path.isabs(pathname):
+            return os.path.join(new_root, pathname)
+        else:
+            return os.path.join(new_root, pathname[1:])
+
+    elif os.name == "nt":
+        (drive, path) = os.path.splitdrive(pathname)
+        if path[0] == "\\":
+            path = path[1:]
+        return os.path.join(new_root, path)
+
+    else:
+        raise InstallationError(
+            f"Unknown platform: {os.name}\n"
+            "Can not change root path prefix on unknown platform."
+        )
+
+
 def get_src_prefix() -> str:
     if running_under_virtualenv():
         src_prefix = os.path.join(sys.prefix, "src")