Python argparse & Unix piping to arguments

Question

Suppose I want the output from rsgen.py to be used as the references argument of my simulate.py script. How can I do it?

In simulate.py

parser.add_argument("references", metavar="RS", type=int, nargs="+", help="Reference string to use")

I tried

# ./simulate.py references < rs.txt 
usage: simulate.py [-h] [--numFrames F] [--numPages P] RS [RS ...]
simulate.py: error: argument RS: invalid int value: 'references'

# ./simulate.py < rs.txt 
usage: simulate.py [-h] [--numFrames F] [--numPages P] RS [RS ...]
simulate.py: error: too few arguments

I believe my piping syntax is wrong, how can I fix it?

Idealy, I want to directly pipe the output from rsgen.py into the references argument of simulate.py

Brian Swift · Accepted Answer · 2012-03-28 07:43:13Z

3

If you mean you want to use the output of rsgen.py as command line parameters to simulate.py, use backquotes which run the contained command and place the output into the command line

    ./simulate.py `./rsgen.py`

answered Mar 28, 2012 at 7:43

Brian Swift

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2 Comments

Manuel Leduc Over a year ago

another syntax is $ ./simulate.py $(rsgen.py) by imho $ ./rsgen.py | ./simulate.py is better.

alexis Over a year ago

This is the simplest and most widely supported shell syntax for turning program output into a commandline argument. You don't need to worry about nesting or anything else with your use case. But don't use this if the output of rsgen is really enormous: In that case, use a pipe and have simulate.py read from stdin.

rahmu · Accepted Answer · 2012-03-28 08:17:57Z

3

If you need to give the output of rsgen.py as an argument, your best solution is to use command substitution. The syntax varies according to the shell you're using, but the following will work on most modern shells:

./simulate.py references $(./rsgen.py)

Side note, Brian Swift's answer uses backticks for command substitution. The syntax is valid on most shells as well, but has the disadvantage of not nesting really well.

On the other hand, if you want to pipe the output of a script into another, you should read from sys.stdin

Example:

a.py

print "hello world"

b.py

import sys

for i in sys.stdin:
    print "b", i

result:

$ ./a.py | ./b.py
b hello world

answered Mar 28, 2012 at 8:17

rahmu

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1 Comment

RobertG Over a year ago

It would look like this in python 3 import sys [NEWLINE] for line in sys.stdin: [NEWLINE] print("###", line)

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