protobuf/python/google/protobuf/reflection.py

# Protocol Buffers - Google's data interchange format
# Copyright 2008 Google Inc.  All rights reserved.
# https://developers.google.com/protocol-buffers/
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# modification, are permitted provided that the following conditions are
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# This code is meant to work on Python 2.4 and above only.

"""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*.
"""

__author__ = 'robinson@google.com (Will Robinson)'


from google.protobuf.internal import api_implementation
from google.protobuf import message


if api_implementation.Type() == 'cpp':
  from google.protobuf.pyext import cpp_message as message_impl
else:
  from google.protobuf.internal import python_message as message_impl

# The type of all Message classes.
# Part of the public interface, but normally only used by message factories.
GeneratedProtocolMessageType = message_impl.GeneratedProtocolMessageType

MESSAGE_CLASS_CACHE = {}


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.
  """
  if descriptor in MESSAGE_CLASS_CACHE:
    return MESSAGE_CLASS_CACHE[descriptor]

  attributes = {}
  for name, nested_type in descriptor.nested_types_by_name.items():
    attributes[name] = MakeClass(nested_type)

  attributes[GeneratedProtocolMessageType._DESCRIPTOR_KEY] = descriptor

  result = GeneratedProtocolMessageType(
      str(descriptor.name), (message.Message,), attributes)
  MESSAGE_CLASS_CACHE[descriptor] = result
  return result