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I write my scripts in python and run them with cmd by typing in:

C:\> python script.py

Some of my scripts contain separate algorithms and methods which are called based on a flag. Now I would like to pass the flag through cmd directly rather than having to go into the script and change the flag prior to run, I want something similar to:

C:\> python script.py -algorithm=2

I have read that people use sys.argv for almost similar purposes however reading the manuals and forums I couldn't understand how it works.

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  • 1
    Have you checked the argparse module ? Its documentation is quite clear and should get you started. Commented May 23, 2013 at 11:33
  • @PierreGM I hadn't seen this before, does it mean that I can add parser = argparse.ArgumentParser() and parser.add_argument(('--Algorithm") and args = parser.parse_args() in my script and then in cmd type in C:\> python script.py --Algorithm=2 so that Algorithm is set to 2 and the python script will run tasks associated with algoritm 2? Commented May 23, 2013 at 11:54

3 Answers 3

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24

There are a few modules specialized in parsing command line arguments: getopt, optparse and argparse. optparse is deprecated, and getopt is less powerful than argparse, so I advise you to use the latter, it'll be more helpful in the long run.

Here's a short example:

import argparse

# Define the parser
parser = argparse.ArgumentParser(description='Short sample app')

# Declare an argument (`--algo`), saying that the 
# corresponding value should be stored in the `algo` 
# field, and using a default value if the argument 
# isn't given
parser.add_argument('--algo', action="store", dest='algo', default=0)

# Now, parse the command line arguments and store the 
# values in the `args` variable
args = parser.parse_args()

# Individual arguments can be accessed as attributes...
print args.algo

That should get you started. At worst, there's plenty of documentation available on line (say, this one for example)...

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What if, say, I write a language interpreter, How would I be able to hande an input file path lang file.lang for example...
15

It might not answer your question, but some people might find it usefull (I was looking for this here):

How to send 2 args (arg1 + arg2) from cmd to python 3:

----- Send the args in test.cmd:

python "C:\Users\test.pyw" "arg1" "arg2"

----- Retrieve the args in test.py:
    import sys, getopt
    print ("This is the name of the script= ", sys.argv[0])
    print("Number of arguments= ", len(sys.argv))
    print("all args= ", str(sys.argv))
    print("arg1= ", sys.argv[1])
    print("arg2= ", sys.argv[2])
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3

Try using the getopt module. It can handle both short and long command line options and is implemented in a similar way in other languages (C, shell scripting, etc):

import sys, getopt


def main(argv):

    # default algorithm:
    algorithm = 1

    # parse command line options:
    try:
       opts, args = getopt.getopt(argv,"a:",["algorithm="])
    except getopt.GetoptError:
       <print usage>
       sys.exit(2)

    for opt, arg in opts:
       if opt in ("-a", "--algorithm"):
          # use alternative algorithm:
          algorithm = arg

    print "Using algorithm: ", algorithm

    # Positional command line arguments (i.e. non optional ones) are
    # still available via 'args':
    print "Positional args: ", args

if __name__ == "__main__":
   main(sys.argv[1:])

You can then pass specify a different algorithm by using the -a or --algorithm= options:

python <scriptname> -a2               # use algorithm 2
python <scriptname> --algorithm=2    # ditto

See: getopt documentation

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3 Comments

Thanks for the answer, just a few quick questions, 1. What is the resposibility of if __name__ == "__main__":? 2. this a definition so can I or should I probably save it in a separate python script and then call in my actual script by from xxx import * and then if __name__ == "__main__": main(sys.argv[1:])?
@Kevin Bell, the test for __name__ == "__main__" will return true if the script is run from the command line, rather than imported in the python interpreter, or another script. The design is up to you - if you program is self contained in that single script, then you can just add the __name__ == "__main__" test to allow it to be launched from the command line. Otherwise, if you import the script, you will have to pass in the argv parameters to the main() call.
I had issues with it, don't know why but Thanks anyways. I got what I wanted with Pierre GM suggested. All the best

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