Issue #13959: Introduce importlib.find_loader().

The long-term goal is to deprecate imp.find_module() in favour of this
API, but it will take some time as some APIs explicitly return/use what
imp.find_module() returns.

Lib/importlib/test/test_api.py

@@ -85,6 +85,54 @@ class ImportModuleTests(unittest.TestCase):
         self.assertEqual(b_load_count, 1)
 
 
+class FindLoaderTests(unittest.TestCase):
+
+    class FakeMetaFinder:
+        @staticmethod
+        def find_module(name, path=None): return name, path
+
+    def test_sys_modules(self):
+        # If a module with __loader__ is in sys.modules, then return it.
+        name = 'some_mod'
+        with util.uncache(name):
+            module = imp.new_module(name)
+            loader = 'a loader!'
+            module.__loader__ = loader
+            sys.modules[name] = module
+            found = importlib.find_loader(name)
+            self.assertEqual(loader, found)
+
+    def test_sys_modules_loader_is_None(self):
+        # If sys.modules[name].__loader__ is None, raise ValueError.
+        name = 'some_mod'
+        with util.uncache(name):
+            module = imp.new_module(name)
+            module.__loader__ = None
+            sys.modules[name] = module
+            with self.assertRaises(ValueError):
+                importlib.find_loader(name)
+
+    def test_success(self):
+        # Return the loader found on sys.meta_path.
+        name = 'some_mod'
+        with util.uncache(name):
+            with util.import_state(meta_path=[self.FakeMetaFinder]):
+                self.assertEqual((name, None), importlib.find_loader(name))
+
+    def test_success_path(self):
+        # Searching on a path should work.
+        name = 'some_mod'
+        path = 'path to some place'
+        with util.uncache(name):
+            with util.import_state(meta_path=[self.FakeMetaFinder]):
+                self.assertEqual((name, path),
+                                 importlib.find_loader(name, path))
+
+    def test_nothing(self):
+        # None is returned upon failure to find a loader.
+        self.assertIsNone(importlib.find_loader('nevergoingtofindthismodule'))
+
+
 class InvalidateCacheTests(unittest.TestCase):
 
     def test_method_called(self):
@@ -114,7 +162,7 @@ class InvalidateCacheTests(unittest.TestCase):
 
 def test_main():
     from test.support import run_unittest
-    run_unittest(ImportModuleTests)
+    run_unittest(ImportModuleTests, FindLoaderTests, InvalidateCacheTests)
 
 
 if __name__ == '__main__':