argparse - "expected one argument" when used -: in argument

[Pythass]

BPO	47002
Nosy	@rhettinger, @ericvsmith, @tirkarthi
Files	test.py: Test file for triggering the argparse issue

^{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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assignee = None
closed_at = None
created_at = <Date 2022-03-13.08:29:17.944>
labels = ['build', 'library', '3.10']
title = 'argparse - "expected one argument" when used -: in argument'
updated_at = <Date 2022-03-13.20:39:33.905>
user = 'https://bugs.python.org/Pythass'

bugs.python.org fields:

activity = <Date 2022-03-13.20:39:33.905>
actor = 'paul.j3'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-03-13.08:29:17.944>
creator = 'Pythass'
dependencies = []
files = ['50668']
hgrepos = []
issue_num = 47002
keywords = []
message_count = 5.0
messages = ['415031', '415032', '415036', '415068', '415097']
nosy_count = 5.0
nosy_names = ['rhettinger', 'eric.smith', 'paul.j3', 'xtreak', 'Pythass']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue47002'
versions = ['Python 3.10']

[Pythass]

By using argparse, and by passing a string as argument that has both a dash "-" and colon ":", python returns "expected one argument" error, despite it works when I use only "-" without ":".

An example of code is attached at the ticket.

Examples of triggering "expected one argument" error:
python test.py -u -1:00
python test.py -u -1:

Examples of NON triggering "expected one argument" error:
python test.py -u -1
python test.py -u=-1:00

[ericvsmith]

Here's a simplified reproducer:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-u", "--utc", choices=["-1:00"])
args = parser.parse_args()

I assume this is related to argparse guessing if an argument is a negative number. See https://stackoverflow.com/questions/9025204/python-argparse-issue-with-optional-arguments-which-are-negative-numbers

From a suggestion there, note that
python test.py -u ' -1:00'
doesn't give the "expected one argument" error. I realize that doesn't solve your problem, but it does shed some light on the issue.

I suspect this can't be fixed without breaking other usages of argparse.

[tirkarthi]

Seems related to https://bugs.python.org/issue9334

[Pythass]

The curious aspect is that for:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-u", "--utc", choices=["-1"])
args = parser.parse_args()

it works. But if we use the colon (:) character as:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-u", "--utc", choices=["-1:"])
args = parser.parse_args()

it does not work anymore... I could think maybe the issue could be related to the presence of ":" character together with "-" character.

For example for:
choices=["-:"] does not work
choices=["-"] works
choices=[":"] works
choices=[":-"] works

So, we get the error if the option start with "-" and has inside also ":" character. At this point I don't think it is related strictly to "negative numbers".

[paulj3]

'-1' and '-1.23' are recognized as numbers, and treated as arguments. '-1' requires some special handling because it is allowed as a flag, as in

    parser.add_argument('-1','--one')

'-1:00' on the other hand is no different from a string like '-foo'. Default is to parse it as a flag. If you don't get this error

argument -f: expected one argument

you are likely to get:

error: unrecognized arguments: -1:23

This can probably be closed as a duplicate of:

https://bugs.python.org/issue9334
argparse does not accept options taking arguments beginning with dash (regression from optparse)

[rhettinger]

Closing as a duplicate.