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 items 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 range(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.
# fill in this function
def fib():
pass # this is a null statement which does nothing when executed, useful as a placeholder.
# testing code
import types
if type(fib()) == types.GeneratorType:
print("Good, The fib function is a generator.")
counter = 0
for n in fib():
print(n)
counter += 1
if counter == 10:
break
# fill in this function
def fib():
a, b = 1, 1
while 1:
yield a
a, b = b, a + b
# testing code
import types
if type(fib()) == types.GeneratorType:
print("Good, The fib function is a generator.")
counter = 0
for n in fib():
print(n)
counter += 1
if counter == 10:
break
test_output_contains("Good, The fib function is a generator.")
success_msg("Excellent!")