import os
import contractions

directory = 'C:\Users\User\Desktop\test1'
os.chdir('C:\Users\User\Desktop\test1')

for filename in os.listdir(directory):
    if filename.endswith(".txt"):
        f = open(filename, 'r')
        text = f.read()

        lines = [contractions.fix() for line in filename]
        with open(filename, 'w') as out:
            out.writelines(lines)

Traceback (most recent call last):
  File "C:UsersUserDesktopreplace contractions.py", line 15, in <module>
    lines = [contractions.fix() for line in filename]
  File "C:UsersUserDesktopreplace contractions.py", line 15, in <listcomp>
    lines = [contractions.fix() for line in filename]
TypeError: fix() missing 1 required positional argument: 's'