[Python-Dev] can't assign to function call

Steven D'Aprano steve at pearwood.info
Mon Mar 18 16:40:37 CET 2013

On 19/03/13 02:01, Hrvoje Niksic wrote:
> On 03/18/2013 03:23 PM, Chris Angelico wrote:
>> The languages that permit you to assign to a function call all have
>> some notion of a reference type.
> Assigning to function calls is orthogonal to reference types.  For example, Python manages assignment to subscripts without having references just fine:
> val = obj[index]      # val = obj.__getitem__(index)
> obj[index] = val      # obj.__setitem__(index, val)
> In analogy with that, Python could implement what looks like assignment to function call like this:
> val = f(arg)          # val = f.__call__(arg)
> f(arg) = val          # f.__setcall__(arg, val)

That's all very well, but what would it do? It's not enough to say that the syntax could exist, we also need to have semantics. What's the use-case here? (That question is mostly aimed at the original poster.)

Aside: I'd reverse the order of the arg, val in any such hypothetical __setcall__, so as to support functions with zero or more arguments:

f(*args, **kwargs) = val  <=>  f.__setcall__(val, *args, **kwargs)


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