Convert any number from base r1 to base r2 in python where r1, r2 in [2, 36]

gistfile1.py

def change_base(n, r1, r2):
    if n == 0:
        return 0
    n = to_base10(n, r1)
    digits = ""
    while n:
        digit = int(n % r2)
        if digit > 9:
            digit = str(chr(65+digit-10))
        digits += str(digit)
        n //= r2
    return digits[::-1]
    
def to_base10(n, b):
    k = str(n)[::-1]
    s = 0
    for a1, a2 in enumerate(k):
        if ord(a2) >=65:
            a2 = 10 + ord(a2)- 65
        s += int(a2) * b ** int(a1)
    return s
    
# input number can be in base 16 e.g 'FF' so  it should be in string format.
n = input("number : ")
r1 = int(input("r1 : "))
r2 = int(input("r2 : "))
print(change_base(n, r1, r2))