Consider the following code:

from argparse import ArgumentParser

parser = ArgumentParser()
parser.add_argument("--clear-magic", action="store_true")
print(parser.parse_args(["--clear"]))

parser.add_argument("--clear", action="store_true")
print(parser.parse_args(["--clear"]))

This has the following output:

Namespace(clear_magic=True)
Namespace(clear=True, clear_magic=False)

I find it surprising and confusing why the clear magic option is accepted when clear is not defined. I'm tempted to say it's a bug. This unstable behaviour is very surprising when iteratively building up the parser.  

Discovered with https://github.com/pypa/virtualenv/issues/1824#issuecomment-627919033

Do you know about this section of the docs?
https://docs.python.org/3/library/argparse.html#argument-abbreviations-prefix-matching

I did not, nm then. Thanks for the link.

One thing that's interesting is that you don't get the "ambiguous option" error if what's provided is an exact match (even if a prefix of both).

For example, if you instead parsed "--clea", it would give the error:

> error: ambiguous option: --clea could match --clear-magic, --clear

I wish the abbreviation behavior was opt-in instead of opt-out, but such is life.

Closing as not a bug.