Subtract values for params in one file from another and save the result to a new file.

subtract_params.py

#!/usr/bin/env python3

import argparse

def read(f):
    """Reads the contents of param=value file and returns a dict"""
    d = {}

    with open(f,'r') as fin:
        for item in fin.readlines():
            s = item.split('=')
            d[s[0]] = int(s[1])

    return d

def write(f, d):
    """Writes dict d to param=value file 'f'"""

    with open(f, 'w') as fout:
        for k,v in sorted(d.items()):
            fout.write('{}={}\n'.format(k,v))

def subtract(d1, d2):
    """
    Subtracts values in d2 from d1 for all params and returns the result in result.
    If an item is present in one of the inputs and not the other, it is added to
    d3.
    """
    # Create a set of all possible keys
    all_keys = set(d1.keys())
    all_keys.update(d2.keys())

    result = {}

    for key in all_keys:
        if key not in d1:
            result[key] = d2[key]
        elif key not in d2:
            result[key] = d1[key]
        else:
            result[key] = d1[key] - d2[key]

    return result

if __name__ == "__main__":

    parser = argparse.ArgumentParser()
    parser.add_argument('file1', type=str, help='path to file 1')
    parser.add_argument('file2', type=str, help='path to file 2')
    parser.add_argument('outfile', type=str, help='path to output file')

    args = parser.parse_args()

    d1 = read(args.file1)
    d2 = read(args.file2)

    d3 = subtract(d1,d2)

    write(args.outfile, d3)

This script does account for parameters that may be in only one of the input files.