Practice: Fibonacci

In the Fibonacci series, each number is the sum of the two before it:

0 1 1 2 3 5 8 13 21 ...

Keep two variables for the last two numbers and move them forward each time:

a, b = 0, 1
a, b = b, a + b    # now a = 1, b = 1
a, b = b, a + b    # now a = 1, b = 2

a, b = b, a + b updates both at the same time, so you don't need a temporary variable.