Palindrome program - I can't figure it out.

[Gabar112]

from string import ascii_letters, digits

s = open("saxo.txt").read()
s = s.lower()
s = "".join([c for c in s if c in ascii_letters or c in digits])

def is_palindrome(s):
    if len(ascii_letters) >=7 and ascii_letters== ascii_letters[::-1]:
        return ascii_letters
    
print(is_palindrome(s))

# I want to create a program that computes all palindromic substrings of length ≥ 7 from the file.
# But somehow I can't make it work. I'm new to Python