[Tutor] Do _all_ Python builtin methods/functions return "None" IF ...

[Steven D'Aprano]

On Fri, Jan 26, 2018 at 08:56:31PM -0600, boB Stepp wrote:

> ... they are not designed to explicitly return something else?

Yes. In the absence of an explicit return, all Python functions will 
return None.


> The reason I ask is that I almost fell into the following trap:
> 
> <GCE trap>
> py3: a_lst = [0, 1, 2]
> py3: b_lst = a_lst.append(3)
> py3: a_lst, b_lst
> ([0, 1, 2, 3], None)
> 
> Instead of "None" I was expecting "[0, 1, 2, 3]".  Obviously I have a
> GCE (Gross Conceptual Error).
> </GCE trap>
> 
> I almost sent the above as my question, but then I realized (Finally!)
> that lst.append() performs an append operation on lst and returns
> None.  Just like print() returns None.
> 
> So my actual question is:  For these types of methods/functions, is
> Python Both versions 2 and 3) consistent throughout and *always*
> returns None?

Mostly. Python's policy is that functions and methods which operate by 
side-effect, or modify the object in place (such as list.append, insert, 
sort, reverse, etc, or dict.update, clear etc), return None. That is to 
prevent the sort of error where you do this:

alist = [4, 1, 2, 5, 3]
blist = alist.sort()
assert blist == [1, 2, 3, 4, 5]
assert alist == [4, 1, 2, 5, 3]  # this will fail


thinking that list.sort returns a copy.

So in general, such functions all return None. However, there have been 
a few changes, such as file.write method, which used to return None in 
Python 2 but in Python 3 returns the number of characters actually 
written.


-- 
Steve