argparse.ArgumentParser does not raise ArgumentError on parse_args() with missing positional argument

[nihadh-ML]

Bug report

Bug description:

Following Code ends up as exception of type SystemExit when called as python test.py or python test.py -c. The catching is possible only over BaseException. According to documentation the code should raise an ArgumentError exception.

#test.py
parser = argparse.ArgumentParser(exit_on_error=False)
parser.add_argument('command',
                        choices=['call', 'text', 'email'],
                        help='What should be done')
parser.add_argument('--contact',
                        help='How to reach')
try:
    parser.parse_args()
except argparse.ArgumentError as err:
    print('Error happened')

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[hugovk]

I don't follow. With the following:

import argparse
parser = argparse.ArgumentParser(exit_on_error=False)
parser.add_argument('command',
                        choices=['call', 'text', 'email'],
                        help='What should be done')
parser.add_argument('--contact',
                        help='How to reach')
parser.parse_args()

Does result in a ArgumentError:

❯ python test.py
Found existing alias for "python". You should use: "p"
Traceback (most recent call last):
  File "/Users/hugo/github/python/cpython/main/test.py", line 8, in <module>
    parser.parse_args()
    ~~~~~~~~~~~~~~~~~^^
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/argparse.py", line 2001, in parse_args
    args, argv = self.parse_known_args(args, namespace)
                 ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/argparse.py", line 2011, in parse_known_args
    return self._parse_known_args2(args, namespace, intermixed=False)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/argparse.py", line 2044, in _parse_known_args2
    namespace, args = self._parse_known_args(args, namespace, intermixed)
                      ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/argparse.py", line 2337, in _parse_known_args
    raise ArgumentError(None, _('the following arguments are required: %s') %
               ', '.join(required_actions))
argparse.ArgumentError: the following arguments are required: command

But you're catching it and printing a message instead. The program then ends as expected. Same with python test.py -c.

This is the bug tracker for the Python language itself. To further discuss your code, please open a topic on the help forum: https://discuss.python.org/c/help/7

[nihadh-ML]

Maybe we have a misunderstanding. What I wanted to say is that in my try-except I cannot catch the error of type ArgumentError, but of type SystemExit. And this is what I get with the same code which you used for your call:

PS D:\PythonProjects\TestPythonProject> python test.py
usage: test.py [-h] [--contact CONTACT] {call,text,email}
test.py: error: the following arguments are required: command

[hugovk]

With your original code, I can catch ArgumentError. Anyway, please continue at https://discuss.python.org/c/help/7

[gvanrossum]

There's a flag to get the Argument... exception instead of SystemError. Pass exit_on_error=False to ArgumentParser(). 3.12+