Anonymous contributor's avatar
Anonymous contributor
Anonymous contributor's avatar
Anonymous contributor
Published May 4, 2022
Contribute to Docs

In Python, a generator is a function or expression that will process a given iterable one object at a time, on demand. A generator will return a generator object, which can be cast as a list or some other data structure as appropriate.


Generators are a convenient means of employing iterator functionality within the syntax of a function or expression. One of the main advantages of generators is that they evaluate items on demand, which means only one item is in memory at a time in lieu of the entire sequence (as with a list).

A Generator Function

The following code shows the creation of a generator function. In the function definition, the yield statement is used to return or include an item in the final generator object.

def return_evens(lst):
for l in lst:
if l % 2 == 0:
yield l
eggs = [x for x in range(20)]
# Output: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

A Generator Expression

The functionality in the previous example can alternatively be defined as an expression. A generator expression utilizes the same syntax as a list comprehension with parentheses framing the statement instead of square brackets.

eggs = [x for x in range(20)] # a list comprehension
list((x for x in eggs if x % 2 == 0)) # a generator expression

Custom Iteration

A generator object can be incrementally advanced with the next() function. When next() is called the current item is returned and the state is saved.


All contributors

Looking to contribute?

Learn Python on Codecademy