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argparse assertion failure with multiline metavars #77048
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MaT1g3R commented
on Feb 18, 2018 MaT1g3RmannequinMannequinAuthorIf I run this script with -h
-----8<------------------from argparse import ArgumentParser mapping = ['123456', '12345', '12345', '123'] p = ArgumentParser('11111111111111') p.add_argument('-v', '--verbose', help='verbose mode', action='store_true') p.add_argument('targets', help='installation targets', nargs='+', metavar='\n'.join(mapping)) p.parse_args() ---------8<
I get an error:
---------8<--------------------Traceback (most recent call last): File "tmp.py", line 7, in <module> p.parse_args() File "/usr/lib/python3.6/argparse.py", line 1730, in parse_args args, argv = self.parse_known_args(args, namespace) File "/usr/lib/python3.6/argparse.py", line 1762, in parse_known_args namespace, args = self._parse_known_args(args, namespace) File "/usr/lib/python3.6/argparse.py", line 1968, in _parse_known_args start_index = consume_optional(start_index) File "/usr/lib/python3.6/argparse.py", line 1908, in consume_optional take_action(action, args, option_string) File "/usr/lib/python3.6/argparse.py", line 1836, in take_action action(self, namespace, argument_values, option_string) File "/usr/lib/python3.6/argparse.py", line 1020, in __call__ parser.print_help() File "/usr/lib/python3.6/argparse.py", line 2362, in print_help self._print_message(self.format_help(), file) File "/usr/lib/python3.6/argparse.py", line 2346, in format_help return formatter.format_help() File "/usr/lib/python3.6/argparse.py", line 282, in format_help help = self._root_section.format_help() File "/usr/lib/python3.6/argparse.py", line 213, in format_help item_help = join([func(*args) for func, args in self.items]) File "/usr/lib/python3.6/argparse.py", line 213, in <listcomp> item_help = join([func(*args) for func, args in self.items]) File "/usr/lib/python3.6/argparse.py", line 334, in _format_usage assert ' '.join(pos_parts) == pos_usage AssertionError -----8< ```------------------
I don't understand the -h (display help part) but pasting code into repository 3.8 IDLE editor and running, I get same traceback.
Never having used argparse, I don't know if a multiline metavar is expected to work. Assuming so, can you suggest a fix?
MaT1g3R commented
on Feb 24, 2018 MaT1g3RmannequinMannequinAuthorI tried to include line breaks for listing options for a positional argument.
The default metavar for that is something like {opt1, opt2, op3},
however I wanted it to display the options on separate lines.If newlines are not permitted in metavars, the user should see
ValueError: newline not permitted in metavarpaulj3 commented
on Mar 18, 2018 paulj3mannequinMannequinI haven't seen anyone try to use \n in a metavar before, but other special characters like [] and () produce this assertion error. At this point the code is trying split the usage into 2 or more lines, because it's too long for one. It creates a usage for optionals and positionals separately.
In a clumsy way, it formats the whole usage string, and tries to split it into pieces so it can decide to split long lines. The 'assert' is used to make sure it has split the line into meaningful blocks. It splits with
r'\(.*?\)+|\[.*?\]+|\S+'basically white space (including nl) and [] and () which are used to mark optional arguments and groups. So including any of these characters in text via metavar will screwup this split.
We could try to refine this splitting expression, but that feels like a never ending task as users become more inventive.
I suggested a major rewrite of this section, one that keeps the pieces a list, and joins them after deciding how many can fit on a line.
No one has, to my knowledge, come up with a comprehensive list of characters that will cause problems here.
argparse does provide a backup - a user provided usage string. That's not as nice as a automatically generated one, but if you have to have something special, that's the way to go. In the long run there's only so much that general purpose parser can do to accommodate special needs.
It works for me on 3.11:
% ./python.exe b.py -h
usage: 11111111111111 [-h] [-v] 123456
12345
12345
123 [123456
12345
12345
123 ...]positional arguments:
123456
12345
12345
123
installation targetsoptions:
-h, --help show this help message and exit
-v, --verbose verbose mode- added a commit that references this issue
on May 28, 2023 This was fixed some time between Python 3.7.17 and 3.8.19.
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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