To make mypy happy when I have an optional job_config, I'm having to write code like this:

        if job_config is not None:
            query_job = self.bqclient.query(sql, job_config=job_config)
        else:
            query_job = self.bqclient.query(sql)

This if/else should be unnecessary, but mypy complains that job_config argument to query can't be None.

The root cause is that the type annotation is only QueryJobConfig

It should be Optional[QueryJobConfig].

This same issue occurs in copy_table and extract_table.