Python test

test.py

#!/usr/bin/env python

from sys import argv
import os

def read_file (file):
    if os.path.exists(file):
        with open( file , "r") as f:
            words = f.read()
            f.closed
            return words

def write_file ( file, words ):
    with open(file, 'w') as file:
        file.write("\n".join(words))
    

def compare_wordlists( words1, words2 ):
    list1 = words1.splitlines()
    list2 = words2.splitlines()
    onlyleft = []
    onlyright = []
    for word in list1 + list2:
        if word in list1 and word not in list2:
            onlyleft.append(word)
        if word in list2 and word not in list1:
            onlyright.append(word)
    return (onlyleft, onlyright)
    
def main ():
    file1 = argv[1]
    file2 = argv[2]
    words1 = read_file( file1 )
    words2 = read_file( file2 )
    onlyleft, onlyright = compare_wordlists( words1, words2 )
    write_file("python_first_only", onlyleft)
    write_file("python_last_only", onlyright)

if __name__ == '__main__':
    main()

Key suggestions: try pathlib for gentler file handling, and take advantage of list comprehensions.

from pathlib import Path
from sys import argv

def load_words(filename):
   return Path(filename).read_text(encoding="utf-8").splitlines()

def main():
    words1, words2 = [load_words(arg) for arg in argv[1:]]
    comparison = {
        "python_first_only": [word for word in words1 if word not in words2],
        "python_second_only": [word for word in words2 if word not in words1],
    }

    for label, result in comparison.items():
        Path(label).write_text("\n".join(result), encoding="utf-8")

if __name__ == "__main__":
    main()

Also consider sets for your wordlist comparison, if you only want unique words from each.

def load_words(filename):
   return set(Path(filename).read_text(encoding="utf-8").splitlines())

def main():
    words1, words2 = [load_words(arg) for arg in argv[1:]]
    comparison = {
        "python_first_only": words1 - words2,
        "python_second_only": words2 - words1,
    }
    # ...