argparse should offer an alternative to SystemExit in case a parse fails

[geophile]

BPO	39333
Nosy	@rhettinger, @ericvsmith, @tirkarthi, @geophile

^{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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GitHub fields:

assignee = None
closed_at = <Date 2020-01-14.20:59:52.338>
created_at = <Date 2020-01-14.20:36:54.250>
labels = ['type-feature', 'library']
title = 'argparse should offer an alternative to SystemExit in case a parse fails'
updated_at = <Date 2020-01-14.20:59:52.337>
user = 'https://github.com/geophile'

bugs.python.org fields:

activity = <Date 2020-01-14.20:59:52.337>
actor = 'geophile'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-14.20:59:52.338>
closer = 'geophile'
components = ['Library (Lib)']
creation = <Date 2020-01-14.20:36:54.250>
creator = 'geophile'
dependencies = []
files = []
hgrepos = []
issue_num = 39333
keywords = []
message_count = 4.0
messages = ['359991', '359994', '359995', '359997']
nosy_count = 5.0
nosy_names = ['rhettinger', 'eric.smith', 'paul.j3', 'xtreak', 'geophile']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39333'
versions = []

[geophile]

If parse_args fails, SystemExit is raised, carrying an exit code of 2, and the help message is printed. For an embedded usage of argparse, this behavior is undesirable.

I am writing an interactive console application, using argparse to parse input. When a parse fails, I would like to print an error message and continue, not terminate the program. Currently, I need to catch SystemExit to be able to do this, which has obvious problems, (e.g., what if something other that argparse is causing the exit?)

I'd like to see some way to specify alternative behavior, e.g. raise an exception of a given type.

[tirkarthi]

Does overriding exit help here?

https://docs.python.org/3.8/library/argparse.html#argparse.ArgumentParser.exit

[ericvsmith]

Maybe argparse could raise an exception derived from SystemExit, then you could catch that.

[geophile]

Yes! I didn't know about that method, thank you.