Calculate digital root with modulo arithmetic.

digital_root.py

#!/usr/bin/env python    


def root(n):             
    """Calculate digital root with modulo arithmetic."""                                             
    return n % 9 or n and 9                       


if __name__ == "__main__":                        
    for i in range(9):   
        assert i == root(i)                       
    for i in range(9, 200*9, 9):                  
        assert 9 == root(i)                       
    for i in range(10, 17):                       
        assert i-9 == root(i)                     
    assert root(123) == 6                         
    assert root(321) == 6                         
    assert root(997) == 7                         
    assert root(7593329) == 2                     

    assert root(root(321) + root(123)) == root(444)