Python 3.9.1 argparse exit_on_error not working in certain situations

Question

I'm writing a REPL that has meta commands. The meta commands can take options and arguments, and I'm trying to use argparse to handle them. As such, I don't want to exit the program entirely if a user doesn't use the command correctly. At most, I'd like to print a usage message and continue the REPL. Here is the basic idea of what I'm trying:

import argparse

p = argparse.ArgumentParser(exit_on_error=False)

p.add_argument('foo')

try:
    p.parse_args([])   # I want to catch the case of a user not supplying a required positional arg
except argparse.ArgumentError:
    print("error detected")

However, running the above snippet gives me this (and exits right after):

usage: test.py [-h] foo
test.py: error: the following arguments are required: foo

The Python 3.9 documentation says this:

Normally, when you pass an invalid argument list to the parse_args() method of an ArgumentParser , it will exit with error info.

If the user would like to catch errors manually, the feature can be enabled by setting exit_on_error to False :

It could be that I'm misunderstanding what an "invalid argument list" means (and the example given in the docs certain works), but I would think that an empty argument list would apply in this case.

I realize there are other ways to handle this (and I've successfully tested one or two of them), but I just want to make sure that I'm not missing something with the exit_on_error parameter. That said, does anyone have any idea why this is not working?

Answer 1

That parameter deals with only one category of error, that raised with handling a particular argument. The test for required arguments is handled differently, and isn't diverted by this parameter. I suspect this has been raised on the Python bug/issues, but I haven't paid a lot of attention to it. Changing the parser.error or parser.exit methods works for both categories of error.