Using argparse to choose a function AND supply options

Question

As described in the accepted answer to Most pythonic way of accepting arguments using optparse, I have a program with a function that performs operations on a string. The program uses argparse to check whether the string is provided as-is or in a file and massages the input as needed to pass it to the function.

Now I want to extend the program with a more advanced version of my function, but still leave the basic version in for comparison, somewhat as in Use argparse to run 1 of 2 functions in my script. Where I believe my situation differs is that regardless of the function that gets called, I want the option of also passing my existing input flags.

Just adding a new argument to the parser and nesting my previous code inside a if/else that checks for that flag doesn't work: it complains about the wrong number of arguments. I am aware of sub-commands, but I am still pretty new with argparse and it just seems like that would be overkill for what I want - but maybe not.

tl;dr: I need to choose one of two functions and one of two input types; both input types apply to both functions. Thanks for any help!

Edited to add code:

p = argparse.ArgumentParser(description="program.py")
p.add_argument("-e", dest='extended')   #The new flag causing the trouble
p.add_argument("-s", dest="string")
p.add_argument("-f", dest="infile")

args = p.parse_args()

if args.extended:
    if args.infile:
        with open(args.infile,'r') as f:
            for line in enumerate(f.readlines()):
                print 'Input: ', line[1],    
                output = funcExtended(line[1])  #new and improved function
                print 'Extended output: ', output
    elif args.string:
        output = funcExtended(args.string)
        print output
    else:  #my future default option to grab strings from a database
        print 'This will soon work: extended'
else:   #I fully realize that I shouldn't have to essentially copy and paste here
    if args.infile:
        with open(args.infile,'r') as f:
            for line in enumerate(f.readlines()):
                print 'Input: ', line[1],    
                output = funcBasic(line[1])  #old and tired function
                print 'Basic output: ', output
    elif args.string:
        output = funcBasic(args.string)
        print output
    else:   #my future default option to grab strings from a database
        print 'This will soon work: basic'

This is a command line utility. Issuing

$ python program.py -s 'string'

returns a properly formatted string, as before. But issuing

$ python program.py -s 'string' -e

returns

program.py: error: argument -e: expected one argument

Whew. Thanks again to anybody who can help!

Could you share the code that generates the error, including the traceback? I'm sure we can help you better still if you do! — Martijn Pieters, Sep 14 '12 at 06:41

score 4 · Accepted Answer · answered Sep 14 '12 at 07:33

If you change your extended argument to a boolean flag with

p.add_argument("-e", dest='extended', action="store_true")

it will no longer expect an argument. You can then invoke your script with

$ python program.py -e -s 'string'

Finally as a bonus here are some ideas to make your code less redundant:

import argparse

def funcExtended(line):
   return " ".join(line)

def funcBasic(line):
    return line.upper()

p = argparse.ArgumentParser(description="program.py")
p.add_argument("-e", "--extended", dest="func", action="store_const", const=funcExtended, default=funcBasic)
p.add_argument("-s", "--string")
p.add_argument("-f", "--infile")

args = p.parse_args()

def readlines(args):
    if args.infile:
        with open(args.infile,'r') as f:
            for line in f:
                yield line.rstrip("\n")
    elif args.string:
        yield args.string
    else:  #my future default option to grab strings from a database
        print 'This will soon work: extended'

for line in readlines(args):
    print 'Input: ', line
    output = args.func(line)
    print "Output: ", output

Thank you! I had been messing around with actions, but I guess I have gotten too tired and didn't do the one you proposed. Your optimization code is appreciated too. I hadn't even considered just letting the string input get echoed to the "Input:"/"Output:" paradigm in addition to the batch input. — verbsintransit, Sep 14 '12 at 07:51

Using argparse to choose a function AND supply options

1 Answers1