#!/usr/bin/env python # # Copyright 2007 Google Inc. # # Licensed under the Apache License, Version 2.0 (the "License"); # you may not use this file except in compliance with the License. # You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # See the License for the specific language governing permissions and # limitations under the License. # """Contains a metaclass and helper functions used to create protocol message classes from Descriptor objects at runtime. Recall that a metaclass is the "type" of a class. (A class is to a metaclass what an instance is to a class.) In this case, we use the GeneratedProtocolMessageType metaclass to inject all the useful functionality into the classes output by the protocol compiler at compile-time. The upshot of all this is that the real implementation details for ALL pure-Python protocol buffers are *here in this file*. """ from google.net.proto2.python.internal import api_implementation from google.net.proto2.python.public import descriptor as descriptor_mod from google.net.proto2.python.public import message _FieldDescriptor = descriptor_mod.FieldDescriptor if api_implementation.Type() == 'cpp': from google.net.proto2.python.internal.cpp import cpp_message as message_impl else: from google.net.proto2.python.internal import python_message as message_impl _NewMessage = message_impl.NewMessage _InitMessage = message_impl.InitMessage class GeneratedProtocolMessageType(type): """Metaclass for protocol message classes created at runtime from Descriptors. We add implementations for all methods described in the Message class. We also create properties to allow getting/setting all fields in the protocol message. Finally, we create slots to prevent users from accidentally "setting" nonexistent fields in the protocol message, which then wouldn't get serialized / deserialized properly. The protocol compiler currently uses this metaclass to create protocol message classes at runtime. Clients can also manually create their own classes at runtime, as in this example: mydescriptor = Descriptor(.....) class MyProtoClass(Message): __metaclass__ = GeneratedProtocolMessageType DESCRIPTOR = mydescriptor myproto_instance = MyProtoClass() myproto.foo_field = 23 ... The above example will not work for nested types. If you wish to include them, use reflection.MakeClass() instead of manually instantiating the class in order to create the appropriate class structure. """ _DESCRIPTOR_KEY = 'DESCRIPTOR' def __new__(cls, name, bases, dictionary): """Custom allocation for runtime-generated class types. We override __new__ because this is apparently the only place where we can meaningfully set __slots__ on the class we're creating(?). (The interplay between metaclasses and slots is not very well-documented). Args: name: Name of the class (ignored, but required by the metaclass protocol). bases: Base classes of the class we're constructing. (Should be message.Message). We ignore this field, but it's required by the metaclass protocol dictionary: The class dictionary of the class we're constructing. dictionary[_DESCRIPTOR_KEY] must contain a Descriptor object describing this protocol message type. Returns: Newly-allocated class. """ descriptor = dictionary[GeneratedProtocolMessageType._DESCRIPTOR_KEY] bases = _NewMessage(bases, descriptor, dictionary) superclass = super(GeneratedProtocolMessageType, cls) new_class = superclass.__new__(cls, name, bases, dictionary) return new_class def __init__(cls, name, bases, dictionary): """Here we perform the majority of our work on the class. We add enum getters, an __init__ method, implementations of all Message methods, and properties for all fields in the protocol type. Args: name: Name of the class (ignored, but required by the metaclass protocol). bases: Base classes of the class we're constructing. (Should be message.Message). We ignore this field, but it's required by the metaclass protocol dictionary: The class dictionary of the class we're constructing. dictionary[_DESCRIPTOR_KEY] must contain a Descriptor object describing this protocol message type. """ descriptor = dictionary[GeneratedProtocolMessageType._DESCRIPTOR_KEY] _InitMessage(descriptor, cls) superclass = super(GeneratedProtocolMessageType, cls) superclass.__init__(name, bases, dictionary) setattr(descriptor, '_concrete_class', cls) def ParseMessage(descriptor, byte_str): """Generate a new Message instance from this Descriptor and a byte string. Args: descriptor: Protobuf Descriptor object byte_str: Serialized protocol buffer byte string Returns: Newly created protobuf Message object. """ result_class = MakeClass(descriptor) new_msg = result_class() new_msg.ParseFromString(byte_str) return new_msg def MakeClass(descriptor): """Construct a class object for a protobuf described by descriptor. Composite descriptors are handled by defining the new class as a member of the parent class, recursing as deep as necessary. This is the dynamic equivalent to: class Parent(message.Message): __metaclass__ = GeneratedProtocolMessageType DESCRIPTOR = descriptor class Child(message.Message): __metaclass__ = GeneratedProtocolMessageType DESCRIPTOR = descriptor.nested_types[0] Sample usage: file_descriptor = descriptor_pb2.FileDescriptorProto() file_descriptor.ParseFromString(proto2_string) msg_descriptor = descriptor.MakeDescriptor(file_descriptor.message_type[0]) msg_class = reflection.MakeClass(msg_descriptor) msg = msg_class() Args: descriptor: A descriptor.Descriptor object describing the protobuf. Returns: The Message class object described by the descriptor. """ attributes = {} for name, nested_type in descriptor.nested_types_by_name.items(): attributes[name] = MakeClass(nested_type) attributes[GeneratedProtocolMessageType._DESCRIPTOR_KEY] = descriptor return GeneratedProtocolMessageType(str(descriptor.name), (message.Message,), attributes)