Why does this argparse code behave differently between Python 2 and 3?

Question

The following code, using argparse's subparsers, fails on Python 3 but runs as expected in Python 2. After comparing the docs, I still can't tell why.

#!/usr/bin/env python
from __future__ import print_function
from argparse import ArgumentParser


def action(args):
    print(args)

if __name__ == '__main__':
    std = ArgumentParser(add_help=False)
    std.add_argument('standard')

    ap = ArgumentParser()
    sp = ap.add_subparsers()

    cmd = sp.add_parser('subcommand', parents=[std], description='Do subcommand')
    cmd.add_argument('arg')
    cmd.set_defaults(do=action)

    args = ap.parse_args()
    args.do(args)

The output from Python 2.7.6 is:

me@computer$ python test.py 
usage: test.py [-h] {subcommand} ...
test.py: error: too few arguments

In Python 3.3.5, I get:

me@computer$ python3 test.py 
Traceback (most recent call last):
  File "test.py", line 21, in <module>
    args.do(args)
AttributeError: 'Namespace' object has no attribute 'do'

Humfrey

Note that if you use a subparser: args = cmd.parse_args() it would work. – Barratry

This code seems to give the same error for me on Python 2.7.4. Most likely you are running the wrong file version or something. It should not work. Try it more carefully again. – Potful

When I type your code in at a Python 2.7.6 interpreter, I get the same error at the line args = ap.parse_args(). – Dissected

Answer 1

the latest argparse release changed how it tested for required arguments, and subparsers fell through the cracks. They are no longer 'required'. http://bugs.python.org/issue9253#msg186387

When you get test.py: error: too few arguments, it's objecting that you did not give it a 'subcommand' argument. In 3.3.5 it makes it past that step, and returns args.

With this change, 3.3.5 should behave the same as earlier versions:

ap = ArgumentParser()
sp = ap.add_subparsers(dest='parser')  # dest needed for error message
sp.required = True   # force 'required' testing

Note - both dest and required need to be set. dest is needed to give this argument a name in the error message.

---

This error:

AttributeError: 'Namespace' object has no attribute 'do'

was produced because the cmd subparser did not run, and did not put its arguments (default or not) into the namespace. You can see that effect by defining another subparser, and looking at the resulting args.

Stereoscope

Thanks, that nailed it. In case there was any wonder, this solution is backward-compatible with Python 2.7 – Humfrey

python 3.8.2. I tried to set the required attribute, but got a traceback instead of a friendly help message. File "/usr/lib/python3.8/argparse.py", line 2035, in _parse_known_args ', '.join(required_actions)) TypeError: sequence item 0: expected str instance, NoneType found – Lydalyddite

@tobixen, see also https://mcmap.net/q/203310/-argparse-with-required-subparser/901925. Did you set the dest? – Stereoscope

No, didn't try dest (hm, it's written plainly in the answer above that dest is required, strange I didn't see it). I tried setting required=True without success. Ended up doing a workaround - github.com/tobixen/calendar-cli/commit/… - perhaps I'm doing this in the wrong way, but ... it works. – Lydalyddite