"""
Write a function, persistence, that takes in a positive parameter num and 
returns its multiplicative persistence, which is the number of times you must 
multiply the digits in num until you reach a single digit.

persistence(39) # returns 3, because 3*9=27, 2*7=14, 1*4=4
                 # and 4 has only one digit
                  
persistence(999) # returns 4, because 9*9*9=729, 7*2*9=126,
                  # 1*2*6=12, and finally 1*2=2

persistence(4) # returns 0, because 4 is already a one-digit number

"""


def persistence(n):
    if n < 10: return 0
    p=1
    for i in str(n): p*=int(i)
    return 1 + persistence(p)    

print(persistence(39))