Convert back and forth between strings and ints, make use of sum().
>>> def foo(n):
n = str(n)
if len(n) == 1:
return int(n)
return foo(sum(int(c) for c in n))
>>> foo(1969)
7
>>>
def foo(n):
n = str(n)
if len(n) == 1:
return int(n)
return foo(sum(int(c) for c in n))
2
solved RECURSIVE function that will sum digits of input