Method _resolve_output_field

django/db/models/expressions.py:346–374

Attempt to infer the output type of the expression. As a guess, if the output fields of all source fields match then simply infer the same type here. If a source's output field resolves to None, exclude it from this check. If all sources are None, then an e

(self)

            return

    def _resolve_output_field(self):
        """
        Attempt to infer the output type of the expression.

        As a guess, if the output fields of all source fields match then simply
        infer the same type here.

        If a source's output field resolves to None, exclude it from this
        check. If all sources are None, then an error is raised higher up the
        stack in the output_field property.
        """
        # This guess is mostly a bad idea, but there is quite a lot of code
        # (especially 3rd party Func subclasses) that depend on it, we'd need a
        # deprecation path to fix it.
        sources_iter = (
            source for source in self.get_source_fields() if source is not None
        )
        for output_field in sources_iter:
            for source in sources_iter:
                if not isinstance(output_field, source.__class__):
                    raise FieldError(
                        "Expression contains mixed types: %s, %s. You must "
                        "set output_field."
                        % (
                            output_field.__class__.__name__,
                            source.__class__.__name__,
                        )
                    )
            return output_field

    @staticmethod
    def _convert_value_noop(value, expression, connection):