Python Generators


Generators are functions that behave like iterators, thus can be used in for loops. For example, a generator that produces even numbers will look like:

def even_numbers(upperlimit):
    current = 0
    while current < upperlimit:
        yield current
        current += 2

Generators are also created using a syntax similar to list comprehension:

even_numbers = (n for n in range(0, 50, 2))

Generators are lazy loading, which means that a value is generated only when it's needed. This can best be observed in the even_numbers function above. It is used as shown below:

ev = even_numbers(10)
next(ev) # Prints 0
next(ev) # Prints 2

The computation for the next item only occurs when next is called. After yield, the function pauses, waiting for the next command to be called. The advantage of this is that it improves memory performance, since the values are only generated when needed, versus having them in an list. this can be shown in the example below, where the list is orders of magnitude larger than the generator.

import sys
a = [n for n in range(0, 1000000, 2)]
print(sys.getsizeof(a)) # Outputs 4290016 bytes
b = (n for n in range(0, 1000000, 2))
print(sys.getsizeof(a)) # Outputs 120 bytes

Another advantage is that generators do not make an assumption on the data structure one wants. For example:

s = set(even_numbers(10))
b = list(even_numbers(10))

As seen above, it is really simple to get different data structures when using generators.

Communication can be done with generators using send. For example:

def even_numbers(upper):
    current = 0
    while current < upper:
        new_max = (yield current)
        if new_max is not None:
            upper = new_max
        current += 2

In the above example, the upper limit can be changed while iterating through the generator. This is shown below:

even = even_numbers(20)
for i in even:
    if i == 4:

The generator.send() works similar to next, in that it resumes the generator. This means that at it also produces the next result in the iteration. This is why the result of the even.send(10) is also printed out, otherwise the above operation would skip printing 6.