from the docu I expected at the second call just a return of value and not a second calculation: there is room for improvement, as Elon Musk would say..

class Ddefault:

    def __init__(self):
        vars(self).setdefault('default', self.set_default())
        vars(self).setdefault('default', self.set_default())

    def set_default(self):
        print(vars(self))
        return 'default'

if __name__ == "__main__":
    Ddefault()

>
>
>     def __init__(self):
>         vars(self).setdefault('default', self.set_default())
>         vars(self).setdefault('default', self.set_default())
>

This code is equivalent to

    def __init__(self):
        x = self.set_default()
        vars(self).setdefault('default', x)

        x = self.set_default()
        vars(self).setdefault('default', x)

>
because the argument is evaluated before the call to setdefault() so you
can't optimise anything here.

Right. If you want the value only calculated once, then just call it once.

You might be interested in collections.defaultdict, which takes a factory function, and only calls it as needed.

I have no problem of making my programme run properly (asking first if in dict etc), but I just read the docu of dict.setdefault

setdefault(key[, default]) 
If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

and it clearly reads if the key is.. return its value, with a full stop;
so either the docu is wrong (but proposes exactly the needed shortcut) or python is as it is: come in & find out

The documentation is correct, in Python argument are computed before the
call to a function, not when they are used. You can try other functions
than dict.setdefault() and see that the behaviour is always the same.

Le dim. 8 déc. 2019 à 22:47, da-dada <report@bugs.python.org> a écrit :

>
> da-dada <georg.klingenberg@travedsl.de> added the comment:
>
> I have no problem of making my programme run properly (asking first if in
> dict etc), but I just read the docu of dict.setdefault
>
> setdefault(key[, default])
> If key is in the dictionary, return its value. If not, insert key with a
> value of default and return default. default defaults to None.
>
> and it clearly reads if the key is.. return its value, with a full stop;
> so either the docu is wrong (but proposes exactly the needed shortcut) or
> python is as it is: come in & find out
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue38998>
> _______________________________________
>

my use case is different (I do a loop), but what I expected from the docs (just for fun!)

class Ddefault:

    def __init__(self):
        vars(self).setdefault('default', self.set_default() if not 'default' in vars(self) else self.default)
        vars(self).setdefault('default', self.set_default() if not 'default' in vars(self) else self.default)
        print(vars(self))

    def set_default(self):
        print(vars(self))
        return 'default'

if __name__ == "__main__":
    Ddefault()

may be the coding diverted from the docs after the fixing of issue 13521 and no one dares correcting..
anyway, I will probably keep my old coding (performance, if there is any)