code: def no_use_pep517_callback(option, opt, value, parser):
      
      # Since --no-use-pep517 doesn't accept arguments, the value argument
      # will be None if --no-use-pep517 is passed via the command-line.
      # However, the value can be non-None if the option is triggered e.g.
      # by an environment variable, for example "PIP_NO_USE_PEP517=true".
      if value is not None:
          msg = """A value was passed for --no-use-pep517,
          probably using either the PIP_NO_USE_PEP517 environment variable
          or the "no-use-pep517" config file option. Use an appropriate value
          of the PIP_USE_PEP517 environment variable or the "use-pep517"
          config file option instead.
          """
          raise_option_error(parser, option=option, msg=msg)
  
      # Otherwise, --no-use-pep517 was passed via the command-line.
      parser.values.use_pep517 = False
docs: Process a value provided for the --no-use-pep517 option.
  
      This is an optparse.Option callback for the no_use_pep517 option.