Empty list as a default param - the problem, and my suggested solution

[Martin Di Paola]

I don't know if it is useful but it is an interesting 
metaprogramming/reflection challenge.

You used `inspect` but you didn't take its full potential. Try to see if 
you can simplify your code and see if you can come with a decorator
that does not require special parameters.

>>> from new import NEW
>>> @NEW
... def new_func(a=[]):
...     a.append('new appended')
...     return a
...
>>> new_func()
['new appended']
>>> new_func()
['new appended']

Spoiler - My solution is at 
https://book-of-gehn.github.io/articles/2021/08/14/Fresh-Python-Defaults.html


On Fri, Aug 13, 2021 at 03:44:20PM -0700, guruyaya wrote:
>I am fairly sure all of us know about this python quirk:
>>>> def no_new_func(a=[]):
>...        a.append('new')
>...        return a
>
>>>> no_new_func()
>['new']
>>>> no_new_func()
>['new', 'new']
>>>>
>
>For some time I was bothered about that there's no elegant way to use empty list or dict as a default parameter. While this can be solved like this:
>>>> def no_new_func(a=None):
>...        if a == None:
>                a = []
>...        a.append('new')
>...        return a
>
>I have to say I find this solution very far from the spirit of python. Kinda ugly, and not explicit. So I've decided to try and create a new module, that will try and make, what I think, is a more beautiful and explicit:
>
>>>> from new import NEW
>>>> @NEW.parse
>... def new_func(a=NEW.new([])):
>...     a.append('new appended')
>...     return a
>...
>>>> new_func()
>['new appended']
>>>> new_func()
>['new appended']
>
>I'd like to hear your thoughts on my solution and code. You can find and give your feedback in this project
>https://github.com/guruyaya/new
>If I see that people like this, I will upload it to pip. I'm not fully sure about the name I choose (I thought about the "new" keyword used in JAVA, not sure it applies here as well)
>
>Thanks in advance for your feedback
>Yair
>-- 
>https://mail.python.org/mailman/listinfo/python-list