def __getitem__(self, extension_handle):
    """Returns the current value of the given extension handle."""

    _VerifyExtensionHandle(self._extended_message, extension_handle)

    result = self._extended_message._fields.get(extension_handle)
    if result is not None:
      return result

    if extension_handle.label == FieldDescriptor.LABEL_REPEATED:
      result = extension_handle._default_constructor(self._extended_message)
    elif extension_handle.cpp_type == FieldDescriptor.CPPTYPE_MESSAGE:
      message_type = extension_handle.message_type
      if not hasattr(message_type, '_concrete_class'):
        # pylint: disable=protected-access
        self._extended_message._FACTORY.GetPrototype(message_type)
      assert getattr(extension_handle.message_type, '_concrete_class', None), (
          'Uninitialized concrete class found for field %r (message type %r)'
          % (extension_handle.full_name,
             extension_handle.message_type.full_name))
      result = extension_handle.message_type._concrete_class()
      try:
        result._SetListener(self._extended_message._listener_for_children)
      except ReferenceError:
        pass
    else:
      # Singular scalar -- just return the default without inserting into the
      # dict.
      return extension_handle.default_value

    # Atomically check if another thread has preempted us and, if not, swap
    # in the new object we just created.  If someone has preempted us, we
    # take that object and discard ours.
    # WARNING:  We are relying on setdefault() being atomic.  This is true
    #   in CPython but we haven't investigated others.  This warning appears
    #   in several other locations in this file.
    result = self._extended_message._fields.setdefault(
        extension_handle, result)

    return result
Returns the current value of the given extension handle.
deps/protobuf/python/google/protobuf/internal/extension_dict.py