Replace certain letters from a variable in python

Pattern = input()

x = Lowercase Letter X = Uppercase Letter

Example:

Pattern = xXXXx

How can I replace every "x" with a random lowercase letter from a list that only has lowercase letters and every "X" with an random uppercase letter from a list that only has uppercase letters?

I would like to use these commands to print them out:

secrets.choice(UppercaseLetters)

secrets.choice(LowercaseLetters)

Edit:

Pattern = str(input())
        for i in Pattern:
            str.replace("X",secrets.choice(UppercaseLetters))
        print(Pattern)