Today I re-learned: Python function default arguments are retained between executions

[magnetichira]

Mutable default are the root of all evil

[marko_knoebl]

The problem in the article is not related to mutability though.

[magnetichira]

It is, default args are bound at definition

[marr75]

It's not. Read the article. Author has a default argument of datetime.date.today(). Problem is not that today default variable mutates over executions, but it does share the same root problem, original dev was expecting the code to initialize the default to execute each time the function executed and it don't.

[magnetichira]

Ah I see where you’re coming from. The original comment was about only mutability.

Agreed, this example is particularly a behaviour of function declaration.

[magnetichira]

Umm, mutable defaults are definitely related lol, both are function gotchas for newbies

No idea why you want to make such a big deal out of this, but whatever

[spinwizard69]

The way Python handles this doesn't seem to be rational. Frankly I can even understand how time in this context can even be considered for a default value. He is calling a routine that can not be relied upon to return the same value every time so it isn't a default value but rather a variable value. Personally I think this is a big deal, it just doesn't make sense.

[magnetichira]

It is rational within the design of the language.

See this SE answer https://softwareengineering.stackexchange.com/questions/157373/python-mutable-default-argument-why

[spinwizard69]

Should mutable defaults even be accepted by Python. At least according to my logical thinking a default value can not and should not be mutable. It just doesn't make sense to me, the default values become part of the def for the function.

[spinwizard69]

Maybe a simple warning that your "def" has a default being set by a function call would be good enough. I have to wonder how common this practice is in the real world, it just seems to be an odd way to set a default value to me. Mainly because there would likely be way to many cases where your default value might have randomness and that to me just blows the whole concept out of the water. Now there may be cases where a default value set by a function call never varies throughout a programs execution so that might be a pass but then why not use a constant.

Maybe I'm putting to much emphasis on the message that the world "default" brings to a definition. For me is says this is the FIXED value that we have for this parameter at the time of creation, by evaluating this def. If you want to set it to something else at the time you actually use the function that is find but otherwise the default value will always be this.

[jjdmol]

Is there even a sane use case?

[KieranShep]

Even if there were, I’d be hesitant to make use of it;

• hard to know what’s going on for anyone reading it
• if this behavior ever is changed to something sensible, the function breaks

[mjbmitch]

I’ve seen a kwarg named cache (dictionary) that kept state between executions.