[Python-ideas] dict.setdefault_call(), or API variations thereupon

[Amit Green]

I use this a lot in my code.

Since `setdefault_call` does not exist, here is how I do it:

d = {}
lookup_d = d.get
provide_d = d.setdefault

for i in range(1000000):  # some large loop
l = (lookup_d(somekey)) or (provide_d(somekey, []))
l.append(somevalue)

I am not arguing for or against `.setdefault_call` -- I'm just providing
information, that I use the referenced behavior hundreds of time in my code.

My solution of using `lookup_d(...) or provide_d(...)` is obviously
inefficient in that it has to do two dictionary lookups (in the case that
the `lookup_d` fails).

A `setdefault_call` would be more efficient; though having to create a
lambda function, might offset this efficency.

So the key issue is readability, not efficiency.

On Thu, Nov 1, 2018 at 9:07 PM Alex Shafer <ashafer01 at gmail.com> wrote:

> I'd like to propose an addition to `dict` but I'm not necessarily
> proposing what's written here as the API. When I initially saw the need for
> this myself, I hastily wrote it as:
>
> def setdefault_call(a_dict, key, default_func):
>     try:
>         return a_dict[key]
>     except KeyError:
>         default = default_func()
>         a_dict[key] = default
>         return default
>
> If its not clear, the purpose is to eliminate the overhead of creating an
> empty list or similar in situations like this:
>
> d = {}
> for i in range(1000000):  # some large loop
>      l = d.setdefault(somekey, [])
>      l.append(somevalue)
>
> # instead...
>
> for i in range(1000000):
>     l = d.setdefault_call(somekey, list)
>     l.append(somevalue)
>
> One potential drawback I see to the concept is that I think there will be
> a need to explicitly say "no arguments can get passed into this call".
> Otherwise users may defeat the purpose with constructions like this:
>
> d.setdefault_call("foo", list, ["default value"])
>
> I'd mainly like feedback on this concept overall, and if its liked,
> perhaps an API discussion to follow. Thanks!
>
> PS
>
> Other APIs I've considered for this are a new keyword argument to the
> existing `setdefault()`, or perhaps more radically for Python, a new
> keyword argument to the `dict()` constructor that would get called as an
> implicit default for `setdefault()` and perhaps used in other scenarios
> (essentially defining a type for dict values).
> _______________________________________________
> 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/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20181101/526ea50e/attachment.html>