duplication in list comprehension

[Skaperen]

extend() has a side effect. If the list comp is of a function that only ever returns None, it's almost certainly using a side effect (unless it's a weird NOOP).

what side effect does extend() have, that is not its definition as a method of the list object it is a method of?

i'm still trying to understand how side-effects became an issue here.  i think side-effects are a bad thing. but it certain cases they are suppose to happen (then the argument needs to be why to not use or not have that method, or not have/use some of what it does).