Here's the outline of a good,in my opinion, base script:

import json,sys,argparse,os

def parser(argv=None):
    # if argv is None, uses the sys.argv[1:]
    parser = argparse.ArgumentParser(....)
    ...
    args = parser.parse_args(argv)
    return(args)

def input(args, *data):

    # if data :
    #     datain=data[0]

    if args.input_file is not None: 
        # input_file might be sys.stdin (if '-')   
        data = args.input_file.read()
    # stdin should work for < redirection
    # I don't know if works for pipe
    ...
    return(datain)

def output(args, *datain) :
    if datain :
        datain=datain[0]
    # output_file might be stdout
    ....
    return(dataout)

def main(args):
    datain = input(args, [])
    dataout = output(args, datain)
    return dataout

if __name__ == "__main__":
    args = parser()
    main(args)

This runs the parser only once if called as a script. If imported it is up to the importer script to run this parser.

A parser may be run multiple times, but usually isn't needed - at least not if the Namespace can be passed around. But each call the parser opens the input/output files. Since one file is opened in write mode, the could result in overlapping opens.

The parser might be test with:

args = parser(['-i', 'inputfile.py', ....]

Another script could do

from template import parser, input, output
def main(args):
    ... input
    # do its own thing
    ... output
# etc