Problem using '-' inside an optional argument when using Python Argparse

Question

I am parsing an argument input:

python parser_test.py --p "-999,-99;-9"

I get this error:

parser_test.py: error: argument --p: expected one argument

Is there a particular reason why including '-' in the optional argument

"-999,-99;-9"

throws the error even while within double quotes? I need to be able to include the '-' sign.

Here is the code:

import argparse

def main():
    parser = argparse.ArgumentParser(description='Input command line arguments for the averaging program')
    parser.add_argument('--p', help='input the missing data filler as an integer')
    args = parser.parse_args()

if __name__=='__main__':
    main()

Answer 1

The quotes do nothing to alter how argparse treats the -; the only purpose they serve is to prevent the shell from treating the ; as a command terminator.

argparse looks at all the arguments first and identifies which ones might be options, regardless of what options are actually defined, by checking which ones start with -. It makes an exception for things that could be negative numbers (like -999), but only if there are no defined options that look like numbers.

The solution is to prevent argparse from seeing -999,-99;-9 as a separate argument. Make it part of the argument that contains the -p using the --name=value form.

python parser_test.py --p="-999,-99;-9"

You can also use "--p=-999,-99;-9" or --p=-999,-99\;-9, among many other possibilities for writing an argument that will cause the shell to parse your command line as two separate commands, python parser_test.py --p-999,-99 and -9.