Compute all palindromic substrings of length ≥ 7 of a textfile

Write a program that given a long string (say order 1.000.000 characters), computes all palindromic substrings of length ≥ 7, i.e. substrings spelled identical read forwards and backwards. You can use the below code to read a complete text from a file to a string, convert it to lower case, and remove everything except letters and digits. The file saxo.txt is a local copy of http://www.gutenberg.org/cache/epub/1150/pg1150.txt

from string import ascii_letters, digits

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

def isPalindrome(str):
    def isPal(str):
        if len(str) >= 7:
            return True
        else:
            str[0] == str[-1] and isPal(str[1:-1])

    return isPal(s)


print(isPalindrome(s))

I got this this exercise where I have to compute all palindromic substrings of length ≥ 7 of a textfile. I have used the above codes to show that my string/textfile is a palindrome. I don't know what I have to do now to compute a list of all palindromic substrings of length ≥ 7.

Thanks in advance.

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