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[Python-Dev] Returning None from methods that mutate object state

Greg Ewing greg.ewing at canterbury.ac.nz
Wed May 21 00:29:50 CEST 2014 
Chris Barker wrote:
> Personally, I often miss the ability to chain operations on mutable 
> objects, but I can only imagine that that design decision was made for 
> good reason. However, as I teach Python, I find I have nothing to say 
> other than "that's the way it's done in Python".

Python has better ways of doing many of the things that
method chaining is used for in other languages. In Java,
for example, I often see it used as a somewhat klunky
workaround for the lack of keyword arguments when
initialising objects.

Other than that, it seems to be mainly for stuffing
multiple operations into one line, which is not something
the Python style generally goes in for.

> I suppose the argument could be that for mutable objects, returning None 
> is an indicator that you are a) working with an mutable object, and b) 
> that the method changes the internal state. But the .pop() example 
> demonstrates that a method can both return something meaningful, and 
> change internal state, so I'm not sure it's really a distinction worth 
> making.

It's not just about mutating the object, it's about a
mutating method with a name that could also plausibly
be the name of a non-mutating method. The canonical
example is sort(), which, if you didn't already know,
could equally well be mutating or non-mutating. In
those cases, I think it's worth making it difficult
to get wrong.

This doesn't apply so much to pop(), which sounds
much more like a mutating method than a non-mutating
one, so it's less likely you'll make a mistake.

-- 
Greg
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