Argparse with required subparser
You need to give subparsers
a dest
.
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='cmd')
subparsers.required = True
Now:
1909:~/mypy$ argdev/python3 stack23349349.py
usage: stack23349349.py [-h] {foo} ...
stack23349349.py: error: the following arguments are required: cmd
In order to issue this 'missing arguments' error message, the code needs to give that argument a name. For a positional argument (like subparses), that name is (by default) the 'dest'. There's a (minor) note about this in the SO answer you linked.
One of the few 'patches' to argparse
in the last Python release changed how it tests for 'required' arguments. Unfortunately it introduced this bug regarding subparsers. This needs to be fixed in the next release (if not sooner).
update
If you want this optional subparsers behavior in Py2, it looks like the best option is to use a two stage parser as described in
How to Set a Default Subparser using Argparse Module with Python 2.7
There has been some recent activity in the related bug/issue
https://bugs.python.org/issue9253
update
A fix to this is in the works: https://github.com/python/cpython/pull/3027
ukrutt
Updated on July 08, 2022Comments
-
ukrutt almost 2 years
I'm using Python 3.4, I'm trying to use
argparse
with subparsers, and I want to have a similar behavior to the one in Python 2.x where if I don't supply a positional argument (to indicate the subparser/subprogram) I'll get a helpful error message. I.e., withpython2
I'll get the following error message:$ python2 subparser_test.py usage: subparser_test.py [-h] {foo} ... subparser_test.py: error: too few arguments
I'm setting the
required
attribute as suggested in https://stackoverflow.com/a/22994500/3061818, however that gives me an error with Python 3.4.0:TypeError: sequence item 0: expected str instance, NoneType found
- full traceback:$ python3 subparser_test.py Traceback (most recent call last): File "subparser_test.py", line 17, in <module> args = parser.parse_args() File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1717, in parse_args args, argv = self.parse_known_args(args, namespace) File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1749, in parse_known_args namespace, args = self._parse_known_args(args, namespace) File "/usr/local/Cellar/python3/3.4.0/Frameworks/Python.framework/Versions/3.4/lib/python3.4/argparse.py", line 1984, in _parse_known_args ', '.join(required_actions)) TypeError: sequence item 0: expected str instance, NoneType found
This is my program
subparser_test.py
- adapted from https://docs.python.org/3.2/library/argparse.html#sub-commands:import argparse # sub-command functions def foo(args): print('"foo()" called') # create the top-level parser parser = argparse.ArgumentParser() subparsers = parser.add_subparsers() subparsers.required = True # create the parser for the "foo" command parser_foo = subparsers.add_parser('foo') parser_foo.set_defaults(func=foo) args = parser.parse_args() args.func(args)
Related question: Why does this argparse code behave differently between Python 2 and 3?
-
Pedro over 7 yearsMore information about the argparse bug here. Upvoting that answer seems to help raise its urgency in the Python bug queue. Really, I'm not just farming rep for someone!
-
anthony sottile about 6 yearsUpdate: the fix has been reverted for "compatibility" reasons: bugs.python.org/issue33109 -- guess this bug will live on forever
-
Henrik about 5 yearsSince Python 3.7
required
can be used as argument:parser.add_subparsers(dest='cmd', required=True)
, docs.python.org -
Moodragonx over 2 yearsthe dest parameter is no longer required, only required is :p. As in, you can just specify
parser.add_subparsers(required=True)
if you don't want to give it a named destination. -
Elazar about 2 years@Moodragonx that gives an incomprehensible error message if the argument is missing.