I want Python argparse to throw an exception rather than usage

Question

I don't think this is possible, but I want to handle exceptions from argparse myself.

For example:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', help='foo help', required=True)
try:
    args = parser.parse_args()
except:
    do_something()

When I run it:

$ myapp.py
usage: myapp --foo foo
myapp: error: argument --foo is required

But I want it to fall into the exception instead.

You're not using argparse for what it was designed for, then. What are you wanting to do? — Chris Morgan, Feb 06 '13 at 11:54
It's not a bad thing to change the way an otherwise great library works, just to tune it to a particular project. — joedborg, Feb 06 '13 at 11:59
Actually you can already do that. simply catch `SystemExit` and replace `sys.stderr` with some other object. — Bakuriu, Feb 06 '13 at 12:01
@jdborg: it's not *necessarily* bad, but it often is bad. In this case, I'd like to hear what you're trying to achieve—there may well be a better approach. — Chris Morgan, Feb 06 '13 at 12:05
Depending on what you want to do, it may be better to modify the argument itself. Is the argument to `--foo` optional? Can you define a custom action to handle it? These may be cleaner actions than overriding `argparse`'s decision to exit. — chepner, Feb 06 '13 at 13:01
@ChrisMorgan, If this user wants a server to have a continuous command line that runs on a server (e.g; a Flask server) and he doesn't want the app to stop running after getting an error, how would you do it? — Yoni Melki, Jun 26 '21 at 05:07
@YoniMelki: that’s a completely different question, with *nothing* in common with this question. Search further, or ask a new question if you haven’t found anything you think matches. Comments are not a suitable place for such matters. — Chris Morgan, Jul 05 '21 at 08:33
Python 3.9 added a `exit_on_error` parameter to `ArgumentParser()`, but it does not (yet?) throw an error for a missing required option nor an unrecognized option. See https://bugs.python.org/issue41255. — user650654, Oct 26 '21 at 23:50

score 82 · Accepted Answer · answered Feb 06 '13 at 11:54

82

You can subclass ArgumentParser and override the error method to do something different when an error occurs:

class ArgumentParserError(Exception): pass

class ThrowingArgumentParser(argparse.ArgumentParser):
    def error(self, message):
        raise ArgumentParserError(message)

parser = ThrowingArgumentParser()
parser.add_argument(...)
...

answered Feb 06 '13 at 11:54

Ned Batchelder

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1

Thought it would be something like that. Thanks for the clear answer. – joedborg Feb 06 '13 at 12:03
3

I was hoping to avoid something like this, though I'm glad it's an option. You see, I'm trying to print out the help menu when I get an "error: too few arguments" notification; my argparse setup requires at least one argument to run successfully, but I figure it'd be polite to print out what those commands are without the user having to run the script with the "-h" parameter. Something like: `parser.set_error('too few arguments', method_to_deal_with_error)` I suppose I can always file a bug if I want this feature. Thanks! – NuclearPeon Jul 20 '13 at 00:03

score 27 · Answer 2 · answered Mar 19 '15 at 06:21

27

in my case, argparse prints 'too few arguments' then quit. after reading the argparse code, I found it simply calls sys.exit() after printing some message. as sys.exit() does nothing but throws a SystemExit exception, you can just capture this exception.

so try this to see if it works for you.

    try:
        args = parser.parse_args(args)
    except SystemExit:
        .... your handler here ...
        return

answered Mar 19 '15 at 06:21

Jacob CUI

1,327
15
11

1

Yes, this option is mentioned above in Bakuriu's comment. The unittesting `test_argparse.py` file uses this approach, along with redefining `error` to create a `ErrorRaisingArgumentParser` class. – hpaulj Mar 19 '15 at 20:48
3

the error message is thrown out on stderr. Is there a way to catch it? – Raj Nov 05 '15 at 20:33
1

@Raj You can temporarily redirect stderr to another stream/file object, for example: https://stackoverflow.com/a/6796752/2128769 – umi Oct 30 '17 at 07:38

I want Python argparse to throw an exception rather than usage

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