diff --git a/lib/spack/spack/modules.py b/lib/spack/spack/modules.py
index 6c32937c3cc..d45fdde7035 100644
--- a/lib/spack/spack/modules.py
+++ b/lib/spack/spack/modules.py
@@ -161,13 +161,13 @@ def write(self):
 
         # Let the extendee modify their extensions before asking for
         # package-specific modifications
-        for extendee in self.pkg.extendees:
-            extendee_spec = self.spec[extendee]
-            extendee_spec.package.setup_dependent_package(
-                self.pkg.module, self.spec)
+        spack_env = EnvironmentModifications()
+        for item in self.pkg.extendees:
+            package = self.spec[item].package
+            package.setup_dependent_package(self.pkg.module, self.spec)
+            package.setup_dependent_environment(spack_env, env, self.spec)
 
         # Package-specific environment modifications
-        spack_env = EnvironmentModifications()
         self.spec.package.setup_environment(spack_env, env)
 
         # TODO : implement site-specific modifications and filters
diff --git a/var/spack/repos/builtin/packages/python/package.py b/var/spack/repos/builtin/packages/python/package.py
index 6d9030805b1..f5237c3b578 100644
--- a/var/spack/repos/builtin/packages/python/package.py
+++ b/var/spack/repos/builtin/packages/python/package.py
@@ -105,7 +105,10 @@ def setup_dependent_environment(self, spack_env, run_env, extension_spec):
 
         pythonpath = ':'.join(python_paths)
         spack_env.set('PYTHONPATH', pythonpath)
-        run_env.set('PYTHONPATH', pythonpath)
+
+        # For run time environment set only the path for extension_spec and prepend it to PYTHONPATH
+        if extension_spec.package.extends(self.spec):
+            run_env.prepend_path('PYTHONPATH', os.path.join(extension_spec.prefix, self.site_packages_dir))
 
 
     def setup_dependent_package(self, module, ext_spec):