[package] populate a special attribute on imported modules (#55255)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55255

This allows packaged code to detect whether or not they are used in a
packaged context, and do different things depending on that. An example
where this might be useful is to control dynamic dependency loading
depending on whether or not something is packaged.

Test Plan: Imported from OSS

Reviewed By: Lilyjjo

Differential Revision: D27544245

Pulled By: suo

fbshipit-source-id: 55d44ef57281524b8d9ab890bd387de97f20bd9f

test/package/test_misc.py

@@ -134,6 +134,46 @@ class TestMisc(PackageTestCase):
         regular_src = inspect.getsourcelines(regular_class)
         self.assertEqual(packaged_src, regular_src)
 
+    def test_dunder_package_present(self):
+        """
+        The attribute '__torch_package__' should be populated on imported modules.
+        """
+        import package_a.subpackage
+
+        buffer = BytesIO()
+        obj = package_a.subpackage.PackageASubpackageObject()
+
+        with PackageExporter(buffer, verbose=False) as pe:
+            pe.save_pickle("obj", "obj.pkl", obj)
+
+        buffer.seek(0)
+        pi = PackageImporter(buffer)
+        mod = pi.import_module(
+            "package_a.subpackage"
+        )
+        self.assertTrue(hasattr(mod, "__torch_package__"))
+
+    def test_dunder_package_works_from_package(self):
+        """
+        The attribute '__torch_package__' should be accessible from within
+        the module itself, so that packaged code can detect whether it's
+        being used in a packaged context or not.
+        """
+        import package_a.use_dunder_package as mod
+
+        buffer = BytesIO()
+
+        with PackageExporter(buffer, verbose=False) as pe:
+            pe.save_module(mod.__name__)
+
+        buffer.seek(0)
+        pi = PackageImporter(buffer)
+        imported_mod = pi.import_module(
+            mod.__name__
+        )
+        self.assertTrue(imported_mod.is_from_package())
+        self.assertFalse(mod.is_from_package())
+
 
 if __name__ == "__main__":
     run_tests()