Ternary plus
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From: Martin Drautzburg on 8 Feb 2010 15:59 Just for the hell of it ... I can easily define __plus__() with three parameters. If the last one is optional the + operation works as expected. Is there a way to pass the third argument to "+"
From: Robert Kern on 8 Feb 2010 16:28 On 2010-02-08 14:59 PM, Martin Drautzburg wrote: > Just for the hell of it ... > > I can easily define __plus__() with three parameters. If the last one is > optional the + operation works as expected. Is there a way to pass the > third argument to "+" No. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Steven D'Aprano on 8 Feb 2010 20:49 On Mon, 08 Feb 2010 21:59:18 +0100, Martin Drautzburg wrote: > Just for the hell of it ... > > I can easily define __plus__() with three parameters. If the last one is > optional the + operation works as expected. Is there a way to pass the > third argument to "+" How do you give three operands to a binary operator? Binary operators only have two sides, a left and a right, so you can only fit two operands around them. Mathematicians solve this problem by using functions: add(a, b, c, d) In Python, you can do this: >>> class F: .... def __add__(self, other, foo=None): .... print self, other, foo .... return 1 .... >>> >>> F() + 3 <__main__.F instance at 0xb7f06f4c> 3 None 1 >>> F().__add__(3, 4) <__main__.F instance at 0xb7f06d8c> 3 4 1 but if you do, people will laugh and point at you in the street. *wink* -- Steven
From: Carl Banks on 8 Feb 2010 21:46 On Feb 8, 12:59 pm, Martin Drautzburg <Martin.Drautzb...(a)web.de> wrote: > Just for the hell of it ... > > I can easily define __plus__() with three parameters. If the last one is > optional the + operation works as expected. Is there a way to pass the > third argument to "+" If, for some reason, you wanted to define a type for which it makes sense to "add" three objects, but not two, you can get the effect you want, kind of. (a + b + c) is useful (a + b) is meaningless You can have __add__ return a closure for the first addition, then perform the operation on the second one. Example (untested): class Closure(object): def __init__(self,t1,t2): self.t1 = t1 self.t2 = t2 def __add__(self,t3): # whole operation peformed here return self.t1 + self.t2 + t3 class MySpecialInt(int): def __add__(self,other): return Closure(self,other) I wouldn't recommend it. Just use a function call with three arguments. Carl Banks
From: Martin Drautzburg on 9 Feb 2010 13:47
Carl Banks wrote: > You can have __add__ return a closure for the first addition, then > perform the operation on the second one. Example (untested): > > class Closure(object): > def __init__(self,t1,t2): > self.t1 = t1 > self.t2 = t2 > def __add__(self,t3): > # whole operation peformed here > return self.t1 + self.t2 + t3 > > class MySpecialInt(int): > def __add__(self,other): > return Closure(self,other) > > > I wouldn't recommend it. Just use a function call with three > arguments. That's way cool. <Flash of insight> Of course! - CURRYING!! If you can return closures you can do everything with just single-parameter functions.</Flash of insight> BTW I am not really trying to add three objects, I wanted a third object which controls the way the addition is done. Sort of like "/" and "//" which are two different ways of doing division. Anyways: thanks a lot. |