diff --git a/lib/spack/spack/build_systems/python.py b/lib/spack/spack/build_systems/python.py
index afa36980f76..87d6b324023 100644
--- a/lib/spack/spack/build_systems/python.py
+++ b/lib/spack/spack/build_systems/python.py
@@ -6,7 +6,7 @@
 import os
 import re
 import shutil
-from typing import Optional
+from typing import Optional  # noqa: F401
 
 import llnl.util.filesystem as fs
 import llnl.util.lang as lang
@@ -108,6 +108,9 @@ def view_file_conflicts(self, view, merge_map):
         return conflicts
 
     def add_files_to_view(self, view, merge_map, skip_if_exists=True):
+        if not self.extendee_spec:
+            return super(PythonExtension, self).add_files_to_view(view, merge_map, skip_if_exists)
+
         bin_dir = self.spec.prefix.bin
         python_prefix = self.extendee_spec.prefix
         python_is_external = self.extendee_spec.external