code: def set_credential_file(cred_file, region=None, authenticate=True):
      
      global regions, services
      region = _safe_region(region)
      identity.set_credential_file(cred_file, region=region,
              authenticate=authenticate)
      regions = tuple(identity.regions)
      services = tuple(identity.services.keys())
      connect_to_services(region=region)
docs: Read in the credentials from the supplied file path, and then try to
      authenticate. The file should be a standard config file in one of the
      following formats:
  
      For Keystone authentication:
          [keystone]
          username = myusername
          password = 1234567890abcdef
          tenant_id = abcdef1234567890
  
      For Rackspace authentication:
          [rackspace_cloud]
          username = myusername
          api_key = 1234567890abcdef
  
      If the region is passed, it will authenticate against the proper endpoint
      for that region, and set the default region for connections.