Obligatory apologies if this has been reported before, I could not find it. It is similar to but different from https://bugs.python.org/issue9334.

>>> from argparse import ArgumentParser
>>> p = ArgumentParser()
>>> p.add_argument('-d')
_StoreAction(option_strings=['-d'], dest='d', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>> p.parse_args(['-d', '12'])
Namespace(d='12')
>>> p.parse_args(['-d', '1,2'])
Namespace(d='1,2')
>>> p.parse_args(['-d', '-12'])
Namespace(d='-12')
>>> p.parse_args(['-d', '-1,2'])
usage: [-h] [-d D]
: error: argument -d: expected one argument

As suggested in issue 9334, passing the argument with an equals sign works:
>>> p.parse_args(['-d=-1,2'])
Namespace(d='-1,2')

But I don't think that this is the intended behavior.

I think it's the same issue. A dash argument that is not clearly a number is interpreted as an optional's flag.  With few exceptions, the parser does not examine the contents of the string

It tests the initial character, it tests for space, it tests for numbers (I'm not sure scientific notation is accepted).  That's about it.

Argparse has a minimum of constrains on what is a valid flag string.  For example, the following is ok:

parser.add_argument('-1,2')
# appearing the namespace as: Namespace(**{'1,2': None})