python_message

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(.....)
  factory = symbol_database.Default()
  factory.pool.AddDescriptor(mydescriptor)
  MyProtoClass = factory.GetPrototype(mydescriptor)
  myproto_instance = MyProtoClass()
  myproto.foo_field = 23
  ...
  """

  # 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.

    Raises:
      RuntimeError: Generated code only work with python cpp extension.
    """
    descriptor = dictionary[GeneratedProtocolMessageType._DESCRIPTOR_KEY]

    if isinstance(descriptor, str):
      raise RuntimeError('The generated code only work with python cpp '
                         'extension, but it is using pure python runtime.')

    # If a concrete class already exists for this descriptor, don't try to
    # create another.  Doing so will break any messages that already exist with
    # the existing class.
    #
    # The C++ implementation appears to have its own internal `PyMessageFactory`
    # to achieve similar results.
    #
    # This most commonly happens in `text_format.py` when using descriptors from
    # a custom pool; it calls symbol_database.Global().getPrototype() on a
    # descriptor which already has an existing concrete class.
    new_class = getattr(descriptor, '_concrete_class', None)
    if new_class:
      return new_class

    if descriptor.full_name in well_known_types.WKTBASES:
      bases += (well_known_types.WKTBASES[descriptor.full_name],)
    _AddClassAttributesForNestedExtensions(descriptor, dictionary)
    _AddSlots(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]

    # If this is an _existing_ class looked up via `_concrete_class` in the
    # __new__ method above, then we don't need to re-initialize anything.
    existing_class = getattr(descriptor, '_concrete_class', None)
    if existing_class:
      assert existing_class is cls, (
          'Duplicate `GeneratedProtocolMessageType` created for descriptor %r'
          % (descriptor.full_name))
      return

    cls._decoders_by_tag = {}
    if (descriptor.has_options and
        descriptor.GetOptions().message_set_wire_format):
      cls._decoders_by_tag[decoder.MESSAGE_SET_ITEM_TAG] = (
          decoder.MessageSetItemDecoder(descriptor), None)

    # Attach stuff to each FieldDescriptor for quick lookup later on.
    for field in descriptor.fields:
      _AttachFieldHelpers(cls, field)

    descriptor._concrete_class = cls  # pylint: disable=protected-access
    _AddEnumValues(descriptor, cls)
    _AddInitMethod(descriptor, cls)
    _AddPropertiesForFields(descriptor, cls)
    _AddPropertiesForExtensions(descriptor, cls)
    _AddStaticMethods(cls)
    _AddMessageMethods(descriptor, cls)
    _AddPrivateHelperMethods(descriptor, cls)

    superclass = super(GeneratedProtocolMessageType, cls)
    superclass.__init__(name, bases, dictionary)