protobuf/python/google/protobuf/reflection.py

# Protocol Buffers - Google's data interchange format
# Copyright 2008 Google Inc.  All rights reserved.
# http://code.google.com/p/protobuf/
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following disclaimer
# in the documentation and/or other materials provided with the
# distribution.
#     * Neither the name of Google Inc. nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

# 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 descriptor as descriptor_mod
from google.protobuf import message

_FieldDescriptor = descriptor_mod.FieldDescriptor


if api_implementation.Type() == 'cpp':
  if api_implementation.Version() == 2:
    from google.protobuf.pyext import cpp_message
    _NewMessage = cpp_message.NewMessage
    _InitMessage = cpp_message.InitMessage
  else:
    from google.protobuf.internal import cpp_message
    _NewMessage = cpp_message.NewMessage
    _InitMessage = cpp_message.InitMessage
else:
  from google.protobuf.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.
  """

  # Must be consistent with the protocol-compiler code in
  # proto2/compiler/internal/generator.*.
  _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)