List mutation method gotcha - How well known?

[Lie]

On Mar 14, 4:57 pm, Jarek Zgoda <jzg... at o2.usun.pl> wrote:
> Lie napisa³(a):
>
>
>
> >> foo = [1,2,3,4]
> >> x = foo.append(5)
> >> print x
>
> >> What will be the output (choose one):
>
> >> 1)  [1,2,3,4]
> >> 2)  [1,2,3,4,5]
> >> 3)  That famous picture of Albert Einstein sticking out his tongue
> >> 4)  Nothing - no output
> >> 5)  None of the above
>
> >> I undertake to summarise answers posted to complete this "survey".
>
> > I think I'll choose 3. Well, no, I suppose the correct behavior
> > _should_ be undefined (i.e. what it returns is an implementation
> > details that should not be relied on). The fact that it returns None
> > is just a "coincidence" that happens to happen every time you tested
> > it (you can't prove by ignorance)
>
> I think in Python there's no notion of "void" return type. Deliberate
> choice to return None for functions that modify objects in place seems
> to be OK as long as it is used consistently and documented. Which is the
> case.

No, there is no need for "void" return type, what I meant is that
everything that's not said in the documentation should be assumed to
be an implementation detail, a method or a function that doesn't say
anything about its return type should be assumed to return an
implementation detail (which basically means: Don't rely on this). The
fact that list.append returns none is just a coincidence, you might
encounter another list.append that returns different thing some time
in the future, or the far future, or perhaps at the end of the
galaxy's life.