Python argparse arguments: issue with passing strings containing hyphens

Question

I met a strange behaviour with my python code today. I wrote the following small program to illustrate.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("--list1", "-l1", nargs='+',  help="liste 1",   metavar="THE_LIST")

args = parser.parse_args()

if args.list1:
    print("list1:" + str(args.list1))

I ran the code on a first server. I got the expected behaviour (list1 contains a string that includes '-V').

$ uname -a
Linux computer 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
$ python  --version
Python 2.7.3
$ cat test.py
import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--list1", "-l1",  help="liste 1", metavar="THE_LIST")

args = parser.parse_args()

if args.list1:
    print("list1:" + str(args.list1))
$ python test.py -l1 "abc -V def"
list1:abc -V def

I ran it on a second server (Centos 6.6) and got an error. The hyphen seems to be considered as part of an additional argument although the value for -l1 is still enclosed with double quotes... Any idea would be really appreciated...

$ uname -a
Linux sacapus 2.6.32-504.12.2.el6.x86_64 #1 SMP Wed Mar 11 22:03:14 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
$ python --version
Python 2.7.9
$ cat test.py 
import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--list1", "-l1",  help="liste 1", metavar="THE_LIST")

args = parser.parse_args()

if args.list1:
    print("list1:" + str(args.list1))
$ python test.py -l1 "abc -V def"
usage: test.py [-h] [--list1 THE_LIST]
test.py: error: unrecognized arguments: -V def

Seems to me like this is much more likely to be an issue with your shell/OS than with Python. Added some tags, please feel free to modify if that helps — Ian Clark
– Ian Clark, Commented Apr 27, 2015 at 18:40
Works for me on Python 2.7.6 using bash, ksh, csh, and fish. — cdarke
– cdarke, Commented Apr 27, 2015 at 19:23
I have to agree with @IanClark; it appears to be a shell issue. I usually run Linux, but happened to be on a Windows machine when I came across this. I can't replicate the error under Windows. — Deacon
– Deacon, Commented Apr 27, 2015 at 19:23

Christian Aichinger · Accepted Answer · 2015-04-27 18:41:20Z

1

This really shouldn't happen. To figure out what's going on, I would include

print(repr(sys.argv))

at the beginning of the script, to isolate if the "abc -V def" string is really passed as a single argument by the shell.

If it arrives in your program as a single argument blame argparse, otherwise your shell is messing up. Either way, this seems like something that should become an entry in CentOS's bug database.

FWIW, I can't reproduce this on Debian Jessie with Python 2.7.9:

greek0@orest:/tmp$ python2.7 a.py -l1 "abc -V def"
list1:['abc -V def']

answered Apr 27, 2015 at 18:41

Christian Aichinger

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9 Comments

dputhier Over a year ago

Adding print(repr(sys.argv)) give me ['test.py', '-l1', 'abc', '-V', 'def'] on the centos server.... It may be related to the Python installation on Centos. I got the same error when using the Python 3.3.5 version installed on this server...

Christian Aichinger Over a year ago

So what does print(repr(sys.argv)) print?

dputhier Over a year ago

I got ['test.py', '-l1', 'abc', '-V', 'def'] on the second server while it gives me the expected output (['test.py', '-l1', 'abc -V def']) on the first.

dputhier Over a year ago

That's it. The python executable was replaced with a shell script that contains: #!/bin/sh LD_LIBRARY_PATH=/Softs/users/Python-2.7.9/binaries/lib/ /Softs/users/Python-2.7.9/binaries/bin/python2.7ld (where python2.7ld is the original python executable)

cdarke Over a year ago

@ChristianAichinger: the simplest was to find out which shell you are using is ps (no parameters).

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