# More on Fibonacci sequence: https://en.wikipedia.org/wiki/Fibonacci_number # Here we're writing generator. What is that? # # Ok, using simple words of mortal generator is an object that produces # result by result saving its state in between yields. # # More delighted explanation tells us that generators are forward iterators that # does not store values, but produce them storing its state inbetween of calls. # # Full anwer could be found here: http://stackoverflow.com/a/231855 def fib(n): # We initialte state of iterator here. a, b = 0, 1 while a < n: # Now we're doing some cruel magic, because right now execution of this # function stops, we give execution power back to user yielding him # intermediate result. yield b # But when user will require next value we would start execution from # right here and back to while loop as we have never stop looping around. a, b = b, a + b # Let's lamely calculate sum of first 100 fibonac numbers. sum = 0 for i in fib(100): sum += i print(sum)