How to Set a Default Subparser using Argparse Modu

Question

I'm using Python 2.7 and I'm trying to accomplish a shell like behavior using argparse. My issue, in general, that I cannot seem to find a way, in Python 2.7, to use argparse's subparsers as optional. It's kind of hard to explain my issue so I'll describe what I require from my program.

The program has 2 modes of work:

1. Starting the program with a given command (each command has it's own additional arguments) and additional arguments will run a specific task.
2. Starting the program without a command will start a shell-like program that can take a line of arguments and process them as if the program was called with the given line as it's arguments.

So, if for example my program supports 'cmd1' and 'cmd2' commands, I could use it like so:

• python program.py cmd1 additional_args1
• python program.py cmd2 additional_args2

or with shell mode:

• python program.py
  • cmd1 additional_args1
  • cmd2 additional_args2
  • quit

In addition, I also want my program to be able to take optional global arguments that will effect all commands.

For that I'm using argparse like so (This is a pure example):

parser = argparse.ArgumentParser(description="{} - Version {}".format(PROGRAM_NAME, PROGRAM_VERSION))

parser.add_argument("-i", "--info",  help="Display more information")

subparsers = parser.add_subparsers()

parserCmd1 = subparsers.add_parser("cmd1", help="First Command")
parserCmd1.set_defaults(func=cmd1)

parserCmd2 = subparsers.add_parser("cmd2", help="Second Command")
parserCmd2.add_argument("-o", "--output", help="Redirect Output")
parserCmd2.set_defaults(func=cmd2)

So I can call cmd1 (with no additional args) or cmd2 (with or without -o flag). And for both I can add flag -i to display even more information of the called command.

My issue is that I cannot activate shell mode, because I have to provide cmd1 or cmd2 as an argument (because of using subparsers which are mandatory)

Restrictions:

• I cannot use Python 3 (I know it can be easily done there)
• Because of global optional arguments I cannot check to see if I get no arguments to skip arg parsing.
• I don't want to add a new command to call shell, it must be when providing no command at all

So how can I achieve This kind of behavior with argparse and python 2.7?

Answer 1

Another idea is to use a 2 stage parsing. One handles 'globals', returning strings it can't handle. Then conditionally handle the extras with subparsers.

import argparse

def cmd1(args):
    print('cmd1', args)
def cmd2(args):
    print('cmd2', args)

parser1 = argparse.ArgumentParser()

parser1.add_argument("-i", "--info",  help="Display more information")

parser2 = argparse.ArgumentParser()
subparsers = parser2.add_subparsers(dest='cmd')

parserCmd1 = subparsers.add_parser("cmd1", help="First Command")
parserCmd1.set_defaults(func=cmd1)

parserCmd2 = subparsers.add_parser("cmd2", help="Second Command")
parserCmd2.add_argument("-o", "--output", help="Redirect Output")
parserCmd2.set_defaults(func=cmd2)

args, extras = parser1.parse_known_args()
if len(extras)>0 and extras[0] in ['cmd1','cmd2']:
    args = parser2.parse_args(extras, namespace=args)
    args.func(args)
else:
    print('doing system with', args, extras)

sample runs:

0901:~/mypy$ python stack46667843.py -i info
('doing system with', Namespace(info='info'), [])
0901:~/mypy$ python stack46667843.py -i info extras for sys
('doing system with', Namespace(info='info'), ['extras', 'for', 'sys'])
0901:~/mypy$ python stack46667843.py -i info cmd1
('cmd1', Namespace(cmd='cmd1', func=<function cmd1 at 0xb74b025c>, info='info'))
0901:~/mypy$ python stack46667843.py -i info cmd2 -o out
('cmd2', Namespace(cmd='cmd2', func=<function cmd2 at 0xb719ebc4>, info='info', output='out'))
0901:~/mypy$ 

Answer 2

A bug/issue (with links) on the topic of 'optional' subparsers.

https://bugs.python.org/issue29298

Notice that this has a recent pull request.

---

With your script and the addition of

args = parser.parse_args()
print(args)

results are

1008:~/mypy$ python3 stack46667843.py 
Namespace(info=None)
1009:~/mypy$ python2 stack46667843.py 
usage: stack46667843.py [-h] [-i INFO] {cmd1,cmd2} ...
stack46667843.py: error: too few arguments
1009:~/mypy$ python2 stack46667843.py cmd1
Namespace(func=<function cmd1 at 0xb748825c>, info=None)
1011:~/mypy$ python3 stack46667843.py cmd1
Namespace(func=<function cmd1 at 0xb7134dac>, info=None)

I thought the 'optional' subparsers affected both Py2 and 3 versions, but apparently it doesn't. I'll have to look at the code to verify why.

---

In both languages, subparsers.required is False. If I set it to true

subparsers.required=True

(and add a dest to the subparsers definition), the PY3 error message is

1031:~/mypy$ python3 stack46667843.py
usage: stack46667843.py [-h] [-i INFO] {cmd1,cmd2} ...
stack46667843.py: error: the following arguments are required: cmd

So there's a difference in how the 2 versions test for required arguments. Py3 pays attention to the required attribute; Py2 (apparently) uses the earlier method of checking whether the positionals list is empty or not.

---

Checking for required arguments occurs near the end of parser._parse_known_args.

Python2.7 includes

    # if we didn't use all the Positional objects, there were too few
    # arg strings supplied.
    if positionals:
        self.error(_('too few arguments'))

before the iteration that checks action.required. That's what's catching the missing cmd and saying too few arguments.

So a kludge is to edit your argparse.py and remove that block so it matches the corresponding section of the Py3 version.