Issue #17621: Introduce importlib.util.LazyLoader.

1 files changed, 132 insertions, 0 deletions

diff --git a/Lib/test/test_importlib/test_lazy.py b/Lib/test/test_importlib/test_lazy.py
new file mode 100644
index 0000000..2e191bb
--- /dev/null
+++ b/Lib/test/test_importlib/test_lazy.py
@@ -0,0 +1,132 @@
+import importlib
+from importlib import abc
+from importlib import util
+import unittest
+
+from . import util as test_util
+
+
+class CollectInit:
+
+    def __init__(self, *args, **kwargs):
+        self.args = args
+        self.kwargs = kwargs
+
+    def exec_module(self, module):
+        return self
+
+
+class LazyLoaderFactoryTests(unittest.TestCase):
+
+    def test_init(self):
+        factory = util.LazyLoader.factory(CollectInit)
+        # E.g. what importlib.machinery.FileFinder instantiates loaders with
+        # plus keyword arguments.
+        lazy_loader = factory('module name', 'module path', kw='kw')
+        loader = lazy_loader.loader
+        self.assertEqual(('module name', 'module path'), loader.args)
+        self.assertEqual({'kw': 'kw'}, loader.kwargs)
+
+    def test_validation(self):
+        # No exec_module(), no lazy loading.
+        with self.assertRaises(TypeError):
+            util.LazyLoader.factory(object)
+
+
+class TestingImporter(abc.MetaPathFinder, abc.Loader):
+
+    module_name = 'lazy_loader_test'
+    mutated_name = 'changed'
+    loaded = None
+    source_code = 'attr = 42; __name__ = {!r}'.format(mutated_name)
+
+    def find_spec(self, name, path, target=None):
+        if name != self.module_name:
+            return None
+        return util.spec_from_loader(name, util.LazyLoader(self))
+
+    def exec_module(self, module):
+        exec(self.source_code, module.__dict__)
+        self.loaded = module
+
+
+class LazyLoaderTests(unittest.TestCase):
+
+    def test_init(self):
+        with self.assertRaises(TypeError):
+            util.LazyLoader(object)
+
+    def new_module(self, source_code=None):
+        loader = TestingImporter()
+        if source_code is not None:
+            loader.source_code = source_code
+        spec = util.spec_from_loader(TestingImporter.module_name,
+                                     util.LazyLoader(loader))
+        module = spec.loader.create_module(spec)
+        module.__spec__ = spec
+        module.__loader__ = spec.loader
+        spec.loader.exec_module(module)
+        # Module is now lazy.
+        self.assertIsNone(loader.loaded)
+        return module
+
+    def test_e2e(self):
+        # End-to-end test to verify the load is in fact lazy.
+        importer = TestingImporter()
+        assert importer.loaded is None
+        with test_util.uncache(importer.module_name):
+            with test_util.import_state(meta_path=[importer]):
+                module = importlib.import_module(importer.module_name)
+        self.assertIsNone(importer.loaded)
+        # Trigger load.
+        self.assertEqual(module.__loader__, importer)
+        self.assertIsNotNone(importer.loaded)
+        self.assertEqual(module, importer.loaded)
+
+    def test_attr_unchanged(self):
+        # An attribute only mutated as a side-effect of import should not be
+        # changed needlessly.
+        module = self.new_module()
+        self.assertEqual(TestingImporter.mutated_name, module.__name__)
+
+    def test_new_attr(self):
+        # A new attribute should persist.
+        module = self.new_module()
+        module.new_attr = 42
+        self.assertEqual(42, module.new_attr)
+
+    def test_mutated_preexisting_attr(self):
+        # Changing an attribute that already existed on the module --
+        # e.g. __name__ -- should persist.
+        module = self.new_module()
+        module.__name__ = 'bogus'
+        self.assertEqual('bogus', module.__name__)
+
+    def test_mutated_attr(self):
+        # Changing an attribute that comes into existence after an import
+        # should persist.
+        module = self.new_module()
+        module.attr = 6
+        self.assertEqual(6, module.attr)
+
+    def test_delete_eventual_attr(self):
+        # Deleting an attribute should stay deleted.
+        module = self.new_module()
+        del module.attr
+        self.assertFalse(hasattr(module, 'attr'))
+
+    def test_delete_preexisting_attr(self):
+        module = self.new_module()
+        del module.__name__
+        self.assertFalse(hasattr(module, '__name__'))
+
+    def test_module_substitution_error(self):
+        source_code = 'import sys; sys.modules[__name__] = 42'
+        module = self.new_module(source_code)
+        with test_util.uncache(TestingImporter.module_name):
+            with self.assertRaises(ValueError):
+                module.__name__
+
+
+if __name__ == '__main__':
+    unittest.main()