Python argparse AssertionError

Question

I just started using argparse module. I wrote the following reduced snippet to demonstrate an issue I'm having.

from argparse import ArgumentParser

if __name__ == '__main__':
    parser = ArgumentParser('Test argparse. This string needs to be relatively long to trigger the issue.')
    parser.add_argument('-f', '--fin', help='a', required = True)
    parser.add_argument('-o', '--out ', help='b', required = True)
    parser.add_argument('-t', '--trans', help='c', required = True)

    args = parser.parse_args()
    print(repr(vars(args)))

AssertionError will be produced when script is run with argument -h

Traceback (most recent call last):
  File "arg.py", line 10, in <module>
    args = parser.parse_args()
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1707, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1739, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1945, in _parse_known_args
    start_index = consume_optional(start_index)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1885, in consume_optional
    take_action(action, args, option_string)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1813, in take_action
    action(self, namespace, argument_values, option_string)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 1017, in __call__
    parser.print_help()
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 2341, in print_help
    self._print_message(self.format_help(), file)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 2325, in format_help
    return formatter.format_help()
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 278, in format_help
    help = self._root_section.format_help()
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 208, in format_help
    func(*args)
  File "C:\Users\user\AppData\Local\Continuum\Anaconda\envs\py3k\lib\argparse.py", line 329, in _format_usage
    assert ' '.join(opt_parts) == opt_usage
AssertionError

Reducing the length of the description string passed to ArgumentParser makes it work correctly. Removing one of the arguments will also help.

Am I doing something wrong here? My environment is:

Python 3.3.5 |Anaconda 1.9.2 (64-bit)| (default, Mar 10 2014, 11:25:04) [MSC v.1600 64 bit (AMD64)] on win32

Answer 1

There is an extra space after --out in the code. Change:

parser.add_argument('-o', '--out ', help='b', required = True)

to:

parser.add_argument('-o', '--out', help='b', required = True)

The underlying cause of the problem is an assert check within the Python code that only occurs when Python attempts to break the help text into multiple lines because it is too long. After breaking the text into a list, the Python code joins it back together and compares it to the original to ensure that it is correct. However, the code that breaks the text apart drops the adjoining spaces resulting in a miscompare.

I added prints to the code (argparse.py, Python 2.7):

# wrap the usage parts if it's too long
text_width = self._width - self._current_indent
if len(prefix) + len(usage) > text_width:
    # break usage into wrappable parts
    part_regexp = r'\(.*?\)+|\[.*?\]+|\S+'
    opt_usage = format(optionals, groups)
    pos_usage = format(positionals, groups)
    opt_parts = _re.findall(part_regexp, opt_usage)
    pos_parts = _re.findall(part_regexp, pos_usage)
    print ' '.join(opt_parts)
    print opt_usage
    assert ' '.join(opt_parts) == opt_usage

And the results:

[-h] -f FIN -o OUT -t TRANS
[-h] -f FIN -o OUT  -t TRANS
Traceback (most recent call last):
  File "blah.py", line 9, in <module>
    args = parser.parse_args()

Note the extra space after OUT.

This explains all of the observed behavior:

• Must be long enough to trigger the wrapping behavior.
• Deleting the --trans argument moved --out to the end negating the behavior.
• Deleting the --out argument negateted the behvaior.

Answer 2

I came here with the exact same problem/error, but without having any extra spaces after --out. My problem was that metavar was set to an empty string (metavar=''). Changing that resolved the issue.

Answer 3

The problem is not the extra -h that you've added. Look at the error and the -o argument:

assert ' '.join(opt_parts) == opt_usage

it's joining the whitespace in '--out '. If you remove it, everything should work fine.