Triggering callback on default value in optparse

Question

I'm using Python's optparse to do what it does best, but I can't figure out how to make the option callback trigger on the default argument value if no other is specified via command-line; is this even possible? This would make my code much cleaner.

I can't use argparse unfortunately, as the platform I'm running on has an outdated Python version.

Edit: To provide more detail, I'm adding an option with a callback and a default value

parser.add_option(
  "-f",
  "--format",
  type = "string",
  action = "callback",
  callback = format_callback,
  default = "a,b,c,d")

The callback function is defined as follows:

def format_callback(option, opt, value, parser):
  # some processing
  parser.values.V = processed_value

Basically I'm processing the "--format" value and putting the result into the parser. This works fine, when "--format" is specified directly via command-line, but I'd like the callback to be triggered on the default "a,b,c,d" value as well.

Custommade

Could you provide a little example code, and explain what you're trying to do in code? – Nanji

Note: Using optparse is discouraged since python version 2.7. The optparse module is deprecated and will not be developed further; development will continue with the argparse module. See PEP 0389 for more info. – Attractant

Answer 1

It is simply not possible.

The optparse.OptionParser implementation of parse_args starts with:

def parse_args(self, args=None, values=None):
    """
    parse_args(args : [string] = sys.argv[1:],
               values : Values = None)
    -> (values : Values, args : [string])

    Parse the command-line options found in 'args' (default:
    sys.argv[1:]).  Any errors result in a call to 'error()', which
    by default prints the usage message to stderr and calls
    sys.exit() with an error message.  On success returns a pair
    (values, args) where 'values' is an Values instance (with all
    your option values) and 'args' is the list of arguments left
    over after parsing options.
    """
    rargs = self._get_args(args)
    if values is None:
        values = self.get_default_values()

Default values are set before processing any arguments. Actual values then overwrite defaults as options are parsed; the option callbacks are called when a corresponding argument is found.

So callbacks simply cannot be invoked for defaults. The design of the optparse module makes this very hard to change.

Autotype

Answer 2

You can inject the default when calling parse_args

options, args = parser.parse_args(args=["--option=default"] + sys.argv[1:])

Since flags passed later in the argument list override those passed earlier, this should work. It's possible you may need to modify your callback function to expect this depending on what it is doing.

Myrmecology