What does the output of the Python extend function mean?

Question

What does the return value (None) for the extend function indicate.

common_birds = ["chicken", "blue jay", "crow", "pigeon"]
birds_seen = ["sparrow", "green finch", "gold finch"]

print (common_birds.extend(birds_seen))
# returns None

common_birds.extend(birds_seen)
print (common_birds)
# returns the extended list

It means that the function is not returning anything, since it modifies the list being used to call it — shriakhilc, Jun 03 '19 at 08:54

score 6 · Accepted Answer · answered Jun 03 '19 at 08:54

6

It means that the method doesn't return something, but works "in place" by extending the list. You're not returning a new list, but changing the old one.

answered Jun 03 '19 at 08:54

LukasNeugebauer

1,331
7
10

score 0 · Answer 2 · answered Jun 03 '19 at 08:57

It means that the method does not have a result that you could work with. Instead it has a side effect.

Hopefully the only side effect is the one you'd expect, i.e. that the list is extended.

In your life you'll find void methods that have several side effects, some of which are unwanted.

What does the output of the Python extend function mean?

2 Answers2