On Tue, May 26, 2020 at 2:24 AM Dominik Vilsmeier
<dominik.vilsme...@gmx.de> wrote:
>
> On 25.05.20 17:29, Ricky Teachey wrote:
>
>
> On Mon, May 25, 2020, 6:49 AM Rob Cliffe via Python-ideas
> <python-ideas@python.org> wrote:
>>
>> (Possibly heretical) Thought:
>> ISTM that when the decision was made that arg default values should be
>> evaluated
>> once, at function definition time,
>> rather than
>> every time the function is called and the default needs to be
>> supplied
>> that that was the *wrong* decision.
>> There may have been what seemed good reasons for it at the time (can anyone
>> point me
>> to any relevant discussions, or is this too far back in the Python primeval
>> soup?).
>> But it is a constant surprise to newbies (and sometimes not-so-newbies).
>> As is attested to by the number of web pages on this topic. (Many of them
>> defend
>> the status quo and explain that it's really quite logical - but why does the
>> status quo
>> *need* to be defended quite so vigorously?)
>
>
>
> First of all: supplying a default object one time and having it start fresh
> at every call would require copying the object. But it is not clear what kind
> of copying of these default values should be done. The language doesn't
> inherently know how to arbitrarily make copies of every object; decisions
> have to be made to define what copying the object would MEAN in different
> contexts.
>
> It wouldn't copy the provided default, it would just reevaluate the
> expression. Python has already a way of deferring evaluation, generator
> expressions:
>
> >>> x = 1
> >>> g = (x for __ in range(2))
> >>> next(g)
> 1
> >>> x = 2
> >>> next(g)
> 2
>
> It's like using a generator expression as the default value and then if the
> argument is not provided Python would use `next(gen)` instead of the `gen`
> object itself to fill the missing value. E.g.:
>
> def foo(x = ([] for __ in it.count())): # if `x` is not provided use
> `next` on that generator
> pass
>
> Doing this today would use the generator itself to fill a missing `x`, so
> this doesn't buy anything without changing the language.
>
Well.... if you want to define the semantics that way, there's a way
cleaner form. Just talk about a lambda function:
def foo(x = lambda: []):
pass
and then the function would be called and its return value assigned to
x, if the parameter isn't given.
But if this were actual language syntax, then it would simply be "the
expression is evaluated at call time" or something like that.
ChrisA
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