# Generators

Generators are very easy to implement, but a bit difficult to understand.

Generators are used to create iterators, but with a different approach. Generators are simple functions which return an iterable set of items, one at a time, in a special way.

When an iteration over a set of item starts using the for statement, the generator is run. Once the generator's function code reaches a "yield" statement, the generator yields its execution back to the for loop, returning a new value from the set. The generator function can generate as many values (possibly infinite) as it wants, yielding each one in its turn.

Here is a simple example of a generator function which returns 7 random integers:

``````import random

def lottery():
# returns 6 numbers between 1 and 40
for i in xrange(6):
yield random.randint(1, 40)

# returns a 7th number between 1 and 15
yield random.randint(1,15)

for random_number in lottery():
print "And the next number is... %d!" % random_number
``````

This function decides how to generate the random numbers on its own, and executes the yield statements one at a time, pausing in between to yield execution back to the main for loop.

## Exercise

Write a generator function which returns the Fibonacci series. They are calculated using the following formula: The first two numbers of the series is always equal to 1, and each consecutive number returned is the sum of the last two numbers. Hint: Can you use only two variables in the generator function? Remember that assignments can be done simultaneously. The code

``````a = 1
b = 2
a, b = b, a
``````

will simultaneously switch the values of a and b.