Raise KeyError in Python ServiceDescriptor.FindMethodByName (#9592)

* Align Python version with C-extension version
  (python/google/protobuf/pyext/descriptor.cc)
* Update documentation accordingly
* Add unit test

python/google/protobuf/descriptor.py

@@ -873,11 +873,14 @@ class ServiceDescriptor(_NestedDescriptorBase):
 
     Args:
       name (str): Name of the method.
+
     Returns:
-      MethodDescriptor or None: the descriptor for the requested method, if
-      found.
+      MethodDescriptor: The descriptor for the requested method.
+
+    Raises:
+      KeyError: if the method cannot be found in the service.
     """
-    return self.methods_by_name.get(name, None)
+    return self.methods_by_name[name]
 
   def CopyToProto(self, proto):
     """Copies this to a descriptor_pb2.ServiceDescriptorProto.

python/google/protobuf/internal/descriptor_test.py

@@ -118,6 +118,14 @@ class DescriptorTest(unittest.TestCase):
   def GetDescriptorPool(self):
     return symbol_database.Default().pool
 
+  def testFindMethodByName(self):
+    service_descriptor = (unittest_custom_options_pb2.
+                          TestServiceWithCustomOptions.DESCRIPTOR)
+    method_descriptor = service_descriptor.FindMethodByName('Foo')
+    self.assertEqual(method_descriptor.name, 'Foo')
+    with self.assertRaises(KeyError):
+        service_descriptor.FindMethodByName('MethodDoesNotExist')
+
   def testEnumValueName(self):
     self.assertEqual(self.my_message.EnumValueName('ForeignEnum', 4),
                      'FOREIGN_FOO')