python/google/protobuf/message_factory.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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"""Provides a factory class for generating dynamic messages.

The easiest way to use this class is if you have access to the FileDescriptor
protos containing the messages you want to create you can just do the following:

message_classes = message_factory.GetMessages(iterable_of_file_descriptors)
my_proto_instance = message_classes['some.proto.package.MessageName']()
"""

__author__ = 'matthewtoia@google.com (Matt Toia)'

from google.protobuf import descriptor_pool
from google.protobuf import message
from google.protobuf import reflection


class MessageFactory(object):
  """Factory for creating Proto2 messages from descriptors in a pool."""

  def __init__(self, pool=None):
    """Initializes a new factory."""
    self.pool = pool or descriptor_pool.DescriptorPool()

    # local cache of all classes built from protobuf descriptors
    self._classes = {}

  def GetPrototype(self, descriptor):
    """Builds a proto2 message class based on the passed in descriptor.

    Passing a descriptor with a fully qualified name matching a previous
    invocation will cause the same class to be returned.

    Args:
      descriptor: The descriptor to build from.

    Returns:
      A class describing the passed in descriptor.
    """
    if descriptor not in self._classes:
      descriptor_name = descriptor.name
      if str is bytes:  # PY2
        descriptor_name = descriptor.name.encode('ascii', 'ignore')
      result_class = reflection.GeneratedProtocolMessageType(
          descriptor_name,
          (message.Message,),
          {'DESCRIPTOR': descriptor, '__module__': None})
          # If module not set, it wrongly points to the reflection.py module.
      self._classes[descriptor] = result_class
      for field in descriptor.fields:
        if field.message_type:
          self.GetPrototype(field.message_type)
      for extension in result_class.DESCRIPTOR.extensions:
        if extension.containing_type not in self._classes:
          self.GetPrototype(extension.containing_type)
        extended_class = self._classes[extension.containing_type]
        extended_class.RegisterExtension(extension)
    return self._classes[descriptor]

  def GetMessages(self, files):
    """Gets all the messages from a specified file.

    This will find and resolve dependencies, failing if the descriptor
    pool cannot satisfy them.

    Args:
      files: The file names to extract messages from.

    Returns:
      A dictionary mapping proto names to the message classes. This will include
      any dependent messages as well as any messages defined in the same file as
      a specified message.
    """
    result = {}
    for file_name in files:
      file_desc = self.pool.FindFileByName(file_name)
      for desc in file_desc.message_types_by_name.values():
        result[desc.full_name] = self.GetPrototype(desc)

      # While the extension FieldDescriptors are created by the descriptor pool,
      # the python classes created in the factory need them to be registered
      # explicitly, which is done below.
      #
      # The call to RegisterExtension will specifically check if the
      # extension was already registered on the object and either
      # ignore the registration if the original was the same, or raise
      # an error if they were different.

      for extension in file_desc.extensions_by_name.values():
        if extension.containing_type not in self._classes:
          self.GetPrototype(extension.containing_type)
        extended_class = self._classes[extension.containing_type]
        extended_class.RegisterExtension(extension)
    return result


_FACTORY = MessageFactory()


def GetMessages(file_protos):
  """Builds a dictionary of all the messages available in a set of files.

  Args:
    file_protos: Iterable of FileDescriptorProto to build messages out of.

  Returns:
    A dictionary mapping proto names to the message classes. This will include
    any dependent messages as well as any messages defined in the same file as
    a specified message.
  """
  # The cpp implementation of the protocol buffer library requires to add the
  # message in topological order of the dependency graph.
  file_by_name = {file_proto.name: file_proto for file_proto in file_protos}
  def _AddFile(file_proto):
    for dependency in file_proto.dependency:
      if dependency in file_by_name:
        # Remove from elements to be visited, in order to cut cycles.
        _AddFile(file_by_name.pop(dependency))
    _FACTORY.pool.Add(file_proto)
  while file_by_name:
    _AddFile(file_by_name.popitem()[1])
  return _FACTORY.GetMessages([file_proto.name for file_proto in file_protos])