On Sun, May 24, 2020 at 10:05 PM Dominik Vilsmeier <dominik.vilsme...@gmx.de>
wrote:
> On 24.05.20 18:34, Alex Hall wrote:
>
>
> OK, let's forget the colon. The point is just to have some kind of
> 'modifier' on the default value to say 'this is evaluated on each function
> call', while still having something that looks like `arg=<default>`. Maybe
> something like:
>
> def func(options=from {}):
>
> It looks like the most common use case for this is to deal with mutable
> defaults, so what is needed is some way to specify a default factory,
> similar to `collections.defaultdict(list)` or
> `dataclasses.field(default_factory=list)`. This can be handled by a
> decorator, e.g. by manually supplying the factories or perhaps inferring
> them from type annotations:
>
> @supply_defaults
> def foo(x: list = None, y: dict = None):
> print(x, y) # [], {}
>
>
> @supply_defaults(x=list, y=dict)
> def bar(x=None, y=None):
> print(x, y) # [], {}
>
This still leaves the information out of the signature, which I've argued
is a more important consideration than saving a couple of lines of
boilerplate.
You could solve this by allowing people to write:
```
@supply_defaults(x=list, y=dict)
def bar(x=[], y={}):
print(x, y) # [], {}
```
and telling `supply_defaults` to ignore the actual defaults wherever
factories are supplied, but there's no guarantee people will put sensible
defaults in the signature.
For the mutable defaults only case, you could have:
```
@copy_mutable_defaults
def bar(x=[], y={}):
print(x, y) # [], {}
```
which would only pay attention to a specific whitelist of default types
like list, dict, and set, and copy them each time. Not great.
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/XUAL4Q3OK4TY24TJWOVECC33PTLC266R/
Code of Conduct: http://python.org/psf/codeofconduct/
