CODEWITHSUNDEEP
python-3.xcommand-lineargparse
elly
elly

Reputation: 317

running python3 script in command line

I wrote a function in python which takes 2 lists (with the same length) and returns another list with 2 elements. the function works perfectly but I am trying to run the python script in command line. to do so, I want to use argparse module in python. I wrote the following script in python3 using the following command:

python3 text.py a b results

the script should take 3 arguments 2 lists as input and one list as output. here is the script:

def fun(a, b):
    a_is_greater = 0
    b_is_greater = 0
    for element_a, element_b in zip(a, b):
        if element_a > element_b:
            a_is_greater += 1
        elif element_a < element_b:
            b_is_greater += 1
    return [a_is_greater, b_is_greater]


def main():
    import argparse
    ap = argparse.ArgumentParser(description="")
    ap.add_argument('--list-type', type=list)
    ap.add_argument('--list-type', type=list)
    ap.add_argument('-o', '--outlist', required=True)

    args = ap.parse_args()

    results = fun(a, b)
    return results

if __name__ == "__main__":
    from signal import signal, SIGPIPE, SIG_DFL
    signal(SIGPIPE, SIG_DFL)

    try:
        main()
    except IOError as e:
        if e.errno != 32:
            raise
    except KeyboardInterrupt as e:
        pass

do you know how to fix it? I have tried these 2 lists: a = [4, 5, 2] and b = [3, 5, 4]. fun function in the script works perfectly for these 2 inputs.

Upvotes: 0

Views: 181

Answers (2)

hpaulj
hpaulj

Reputation: 231385

With the corrected code using type=list, sys.argv and args display as:

1353:~/mypy$ python3 stack56531390.py --list1 [4,5,2] --list2 [3,5,4] -o result
['stack56531390.py', '--list1', '[4,5,2]', '--list2', '[3,5,4]', '-o', 'result']
Namespace(list1=['[', '4', ',', '5', ',', '2', ']'], list2=['[', '3', ',', '5', ',', '4', ']'], outlist='result')
[1, 1]

Note that the 'lists' come in as strings. The list function splits that string into a list of characters.

In [607]: list('astring')                                                                              
Out[607]: ['a', 's', 't', 'r', 'i', 'n', 'g']

The fact that fun works for these inputs is more a coincidence, not by design. The inputs don't match the tested: a = [4, 5, 2]' and 'b = [3, 5, 4].

Look at what happens if the user puts spaces in the lists:

1355:~/mypy$ python3 stack56531390.py --list1 [4, 5, 2] --list2 [3,5,4] -o result
usage: stack56531390.py [-h] [--list1 LIST1] [--list2 LIST2] -o OUTLIST
stack56531390.py: error: unrecognized arguments: 5, 2]

or quotes to keep the spaced lists together:

1357:~/mypy$ python3 stack56531390.py --list1 "[4, 5, 2]" --list2 [3,5,4] -o result
['stack56531390.py', '--list1', '[4, 5, 2]', '--list2', '[3,5,4]', '-o', 'result']
Namespace(list1=['[', '4', ',', ' ', '5', ',', ' ', '2', ']'], list2=['[', '3', ',', '5', ',', '4', ']'], outlist='result')
[2, 3]

So while type=list can be made to work, it usually isn't a good choice. Remember, the argparse type is a function that is given a string; it's not a type or class specifier.

If I change the two arguments to take * inputs of type int:

ap.add_argument('--list1', nargs='*', type=int)
ap.add_argument('--list2', nargs='*', type=int)

1358:~/mypy$ python3 stack56531390.py --list1 4 5 2  --list2 3 5 4 -o result
['stack56531390.py', '--list1', '4', '5', '2', '--list2', '3', '5', '4', '-o', 'result']
Namespace(list1=[4, 5, 2], list2=[3, 5, 4], outlist='result')
[1, 1]

Now the two inputs are normal lists of integers, not those lists of characters that include brackets and commas. The comparison will be numeric, not lexical.

Upvotes: 0

Guillaume Fe
Guillaume Fe

Reputation: 369

Is this better?

python3 text.py --list1 [4,5,2] --list2 [3,5,4] -o result # [1, 1] 

def main():
    import argparse

    ap = argparse.ArgumentParser(description="")
    ap.add_argument('--list1', type=list)
    ap.add_argument('--list2', type=list)
    ap.add_argument('-o', '--outlist', required=True)
    args = ap.parse_args()

    results = fun(args.list1, args.list2)
    print(results)
    return results

Code :

def fun(a, b):
    a_is_greater = 0
    b_is_greater = 0
    for element_a, element_b in zip(a, b):
        if element_a > element_b:
            a_is_greater += 1
        elif element_a < element_b:
            b_is_greater += 1
    return [a_is_greater, b_is_greater]


def main():
    import argparse

    ap = argparse.ArgumentParser(description="")
    ap.add_argument('--list1', type=list)
    ap.add_argument('--list2', type=list)
    ap.add_argument('-o', '--outlist', required=True)
    args = ap.parse_args()

    results = fun(args.list1, args.list2)
    print(results)
    return results

if __name__ == "__main__":
    from signal import signal, SIGPIPE, SIG_DFL
    signal(SIGPIPE, SIG_DFL)

    try:
        main()
    except IOError as e:
        if e.errno != 32:
            raise
    except KeyboardInterrupt as e:
        pass

Upvotes: 1

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