On 2020-07-28 at 11:04:25 -0300,
"Joao S. O. Bueno" <jsbu...@python.org.br> wrote:
> Anyway, that is feasible via a decorator.
> Since it can't be done the wya you are proposing as is, since
> having a function as a default argument is valid Python
> (and the function is not called) - and
> having new syntax for this would be more cumbersome
> than using a decorator, I think that closes the gap.
>
> If such a decorator would be useful enough to cut it into the
> stlib, is another question though - I'd probably find it occasionally
> useful myself, but even so, I am +0 on this -
> it is not like
>
> ```
> if parameter is sentinel:
> parameter = factory()
> ```
> would be too much to type.
>
> On a second thought - proper documenting and giving visibility
> to a decorator like this could make it be used in patterns like
> ```
> @factoryargs
> def myfunction(a, b, c=list):
> pass
> ```
> and we could see a drop in the newcomers to Python
> putting a `[]` as default argument.
That's great, until I have something like this:
@factoryargs
def myfunction(a, b, c=list, d=3.4):
pass
Is c a factory? Is d a factory?
> Ok - I just convinced myself - I am +1 for such a decorator now.
I don't mean to be a wet blanket, but what's wrong with a helper
function?
def time_diff_from_now(target_time):
return time_diff(target_time, datetime.datetime.now())
def time_diff(target_time, curr_time):
return curr_time - target_time
No decorators, no new syntax, explicit at the calling sites.
> On Mon, 27 Jul 2020 at 20:42, Richard Damon <rich...@damon-family.org>
> wrote:
>
> > On 7/27/20 10:01 AM, Peter Moore wrote:
> > > I have had a long standing unanswered question on on stackoverflow: is
> > it possible to pass a function to a default parameter so that you could do
> > in essence things like this.
> > >
> > > def time_diff(target_time, curr_time= lambda : datetime.now() ):
> > > return curr_time - target_time
> > >
> > > this would be an syntactical improvement over this style where you have
> > if statement to initialize a missing parameter.
> > >
> > > def time_diff(target_time, curr_time=None):
> > > if curr_time == None:
> > > curr_time = datetime.datetime.now()
> > > return curr_time - target_time
> > I will point out that you CAN pass a function as the default value of a
> > function parameter, and it means that the parameter will be bound to the
> > function itself, so it becomes a callable (so doesn't help you in your
> > case). But this does become an impediment to trying to define it this
> > way, you need somehow to distinguish between the function itself being
> > the default value, or some magically invocation of the function at each
> > call.
> >
> > --
> > Richard Damon
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--
“Whoever undertakes to set himself up as a
judge of Truth and Knowledge is shipwrecked
by the laughter of the gods.” – Albert Einstein
Dan Sommers, http://www.tombstonezero.net/dan
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