Implement the more specific PEP 302 semantics for loaders and what happens upon

load failure in relation to reloads. Also expose
importlib.util.module_for_loader to handle all of the details of this along
with making sure all current loaders behave nicely.

Lib/importlib/test/test_util.py

@@ -0,0 +1,69 @@
+from importlib import util
+from . import util as test_util
+import imp
+import sys
+import types
+import unittest
+
+
+class ModuleForLoaderTests(unittest.TestCase):
+
+    """Tests for importlib.util.module_for_loader."""
+
+    def return_module(self, name):
+        fxn = util.module_for_loader(lambda self, module: module)
+        return fxn(self, name)
+
+    def raise_exception(self, name):
+        def to_wrap(self, module):
+            raise ImportError
+        fxn = util.module_for_loader(to_wrap)
+        try:
+            fxn(self, name)
+        except ImportError:
+            pass
+
+    def test_new_module(self):
+        # Test that when no module exists in sys.modules a new module is
+        # created.
+        module_name = 'a.b.c'
+        with test_util.uncache(module_name):
+            module = self.return_module(module_name)
+            self.assert_(module_name in sys.modules)
+        self.assert_(isinstance(module, types.ModuleType))
+        self.assertEqual(module.__name__, module_name)
+
+    def test_reload(self):
+        # Test that a module is reused if already in sys.modules.
+        name = 'a.b.c'
+        module = imp.new_module('a.b.c')
+        with test_util.uncache(name):
+            sys.modules[name] = module
+            returned_module = self.return_module(name)
+            self.assert_(sys.modules[name] is returned_module)
+
+    def test_new_module_failure(self):
+        # Test that a module is removed from sys.modules if added but an
+        # exception is raised.
+        name = 'a.b.c'
+        with test_util.uncache(name):
+            self.raise_exception(name)
+            self.assert_(name not in sys.modules)
+
+    def test_reload_failure(self):
+        # Test that a failure on reload leaves the module in-place.
+        name = 'a.b.c'
+        module = imp.new_module(name)
+        with test_util.uncache(name):
+            sys.modules[name] = module
+            self.raise_exception(name)
+            self.assert_(sys.modules[name] is module)
+
+
+def test_main():
+    from test import support
+    support.run_unittest(ModuleForLoaderTests)
+
+
+if __name__ == '__main__':
+    test_main()