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.