Wednesday, November 02, 2016

Python Usability Bugs: Formatting argparse subcommands

Suppose you want to build a tool with a simple interface:
usage: sub <command>


  status -  show status
  list   -  print list
Python proposes to use argparse module. And if you follow documentation, the best you can get will be this output:
usage: sub {status,list} ...

positional arguments:
    status       show status
    list         print list
And it you implement proper formatting, your code will look like this:
import argparse
import sys

class CustomHelpFormatter(argparse.HelpFormatter):
  def _format_action(self, action):
    if type(action) == argparse._SubParsersAction:
      # inject new class variable for subcommand formatting
      subactions = action._get_subactions()
      invocations = [self._format_action_invocation(a) for a in subactions]
      self._subcommand_max_length = max(len(i) for i in invocations)

    if type(action) == argparse._SubParsersAction._ChoicesPseudoAction:
      # format subcommand help line
      subcommand = self._format_action_invocation(action) # type: str
      width = self._subcommand_max_length
      help_text = ""
          help_text = self._expand_help(action)
      return "  {:{width}} -  {}\n".format(subcommand, help_text, width=width)

    elif type(action) == argparse._SubParsersAction:
      # process subcommand help section
      msg = '\n'
      for subaction in action._get_subactions():
          msg += self._format_action(subaction)
      return msg
      return super(CustomHelpFormatter, self)._format_action(action)

def check():
  return 0

parser = argparse.ArgumentParser(usage="sub <command>", add_help=False,

subparser = parser.add_subparsers(dest="cmd")
subparser.add_parser('status', help='show status')
subparser.add_parser('list', help='print list')

# custom help messge
parser._positionals.title = "commands"

# hack to show help when no arguments supplied
if len(sys.argv) == 1:

args = parser.parse_args()

if args.cmd == 'list':
elif args.cmd == 'status':

Here you may see the failure of OOP (object oriented programming). The proper answer to this formatting problem is just to define a data structure for command line help in JSON or similar format and let people dump and process it with templates. Once option definition is parsed, the information there is static, so there is no need in those intertwined recursive method calls. So just do it in 2 pass - get dataset and render template.