[Python-ideas] Add "default" kwarg to list.pop()

[Nicolas Rolin]

As a user I always found a bit disurbing that dict pop method have a
default while list and set doesn't.
While it is way more computationally easy to check wether a list or a set
is empty that to check if a key is in a dict, it still create a signature
difference for no real reason (having a default to a built-in in python is
pretty standard).
It would be nice if every built-in/method of built-in type that returns a
value and raise in some case have access to a default instead of raise, and
not having to check the doc to see if it supports a default.

We could for exemple ask ourselves wether or not list.index should have a
default, as it is a method that we explecitely excpect to return a value
and might just raise instead.

2018-10-31 2:08 GMT+01:00 Steven D'Aprano <steve at pearwood.info>:

> On Wed, Oct 31, 2018 at 02:25:25AM +0200, Serhiy Storchaka wrote:
> > 31.10.18 01:44, Giampaolo Rodola' пише:
> > >Sorry in advance if this has been proposed in the past but I couldn't
> > >find anything on python-ideas:
> > >
> > > >>> l = []
> > > >>> l.pop(default=1)
> > >1
> [...]
>
> > It is just
> >
> >     l.pop() if l else default
>
> It might *do* the same thing, but it doesn't communicate the
> programmer's intention as well.
>
> {}.pop('key', default) could be written using LBYL too, but the
> intention is much clearer given an explicit default argument.
>
> The only advantage of the "if l" version is that if the default is
> expensive to calculate, we can short-circuit it.
>
>
> > or
> >
> >     (l or [default]).pop()
>
> That's clever, but it is also wasteful, building a single-item list only
> to immediately pop the item out of it and throw the list away.
>
> [steve at ando ~]$ python3.5 -m timeit -s "l = []" "l.pop() if l else None"
> 10000000 loops, best of 3: 0.0739 usec per loop
>
> [steve at ando ~]$ python3.5 -m timeit -s "l = []" "(l or [None]).pop()"
> 1000000 loops, best of 3: 0.421 usec per loop
>
>
>
> --
> Steve
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>



-- 

--
*Nicolas Rolin* | Data Scientist
+ 33 631992617 - nicolas.rolin at tiime.fr <prenom.nom at tiime.fr>


*15 rue Auber, **75009 Paris*
*www.tiime.fr <http://www.tiime.fr>*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20181031/7a3d3eea/attachment.html>