def IsInitialized(self, errors=None):
    """Checks if all required fields of a message are set.

    Args:
      errors:  A list which, if provided, will be populated with the field
               paths of all missing required fields.

    Returns:
      True iff the specified message has all required fields set.
    """

    # Performance is critical so we avoid HasField() and ListFields().

    for field in required_fields:
      if (field not in self._fields or
          (field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE and
           not self._fields[field]._is_present_in_parent)):
        if errors is not None:
          errors.extend(self.FindInitializationErrors())
        return False

    for field, value in list(self._fields.items()):  # dict can change size!
      if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
        if field.label == _FieldDescriptor.LABEL_REPEATED:
          if (field.message_type.has_options and
              field.message_type.GetOptions().map_entry):
            continue
          for element in value:
            if not element.IsInitialized():
              if errors is not None:
                errors.extend(self.FindInitializationErrors())
              return False
        elif value._is_present_in_parent and not value.IsInitialized():
          if errors is not None:
            errors.extend(self.FindInitializationErrors())
          return False

    return True
Checks if all required fields of a message are set.

    Args:
      errors:  A list which, if provided, will be populated with the field
               paths of all missing required fields.

    Returns:
      True iff the specified message has all required fields set.
    
deps/protobuf/python/google/protobuf/internal/python_message.py