[Python-ideas] Syntactic sugar to declare partial functions

[Steven D'Aprano]

On Mon, Aug 13, 2018 at 07:46:49PM +0200, Stefan Behnel wrote:
> Michel Desmoulin schrieb am 09.08.2018 um 18:59:
> > I'd rather have functools.partial() to be added as a new method on
> > function objects.
[...]
> > add_2 = add.partial(2)
> 
> Except that this only works for functions, not for other callables.
> Meaning, code that uses this for anything but its self-defined functions
> will break as soon as someone passes in a callable object that is not a
> function.

That's an excellent point.

If it were important to cover those use-cases, we could add a partial 
method to types, methods, builtin-functions etc. But I suspect that 
doing that would increase the cost of this proposed feature past the 
point where it is worthwhile.

Do we often call functools.partial on arbitrary callable objects that we 
don't know in advance? (Not a rhetorical question.) I know I don't -- 
all the times I've called partial, I've known what the callable was, and 
it was always a function I created with def. But maybe others do 
something differently.

I don't mind writing this:

    # only a minor inconvenience
    spam = func.partial(arg)
    eggs = functools.partial(callable, arg)

given that I knew upfront that func was a function and callable was not. 
But if I didn't know, the utility of a partial method would be hugely 
reduced, and I'd just use functools.partial for everything.



-- 
Steve