>>> # Fibonacci series: ... # the sum of two elements defines the next ... a, b = 0, 1 >>> while b < 10: ... print b ... a, b = b, a+b ... 1 1 2 3 5 8 >>> a, b = 0, 1 >>> while b < 10: print b a = b b = a+b 1 2 4 8 Why a, b = b, a+b isn't a = b; b = a+b ?