code: def get_pull_request_number(task, source_env_prefix):
      
      pull_request = _extract_from_env_in_payload(task, source_env_prefix + '_PULL_REQUEST_NUMBER')
      if pull_request is not None:
          pull_request = int(pull_request)
      return pull_request
docs: Get what Github pull request created the graph.
  
      Args:
          obj (ChainOfTrust or LinkOfTrust): the trust object to inspect
          source_env_prefix (str): The environment variable prefix that is used
              to get repository information.
  
      Returns:
          int: the pull request number.
          None: if not defined for this task.