blob: 839f9ade7acdee961bcf053e0a6980d5688f4d3a [file] [log] [blame]
#!/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':
if api_implementation.Version() == 2:
from google.net.proto2.python.internal.cpp import cpp_message
_NewMessage = cpp_message.NewMessage
_InitMessage = cpp_message.InitMessage
else:
from google.net.proto2.python.internal import cpp_message
_NewMessage = cpp_message.NewMessage
_InitMessage = cpp_message.InitMessage
else:
from google.net.proto2.python.internal import python_message
_NewMessage = python_message.NewMessage
_InitMessage = python_message.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)
setattr(descriptor, '_concrete_class', new_class)
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)
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)