Param decorator - can you suggest improvements

[Steven D'Aprano]

On Fri, 18 Jan 2013 03:38:08 +0000, Dan Sommers wrote:

> On Thu, 17 Jan 2013 15:21:08 +0000, Steven D'Aprano wrote:
> 
>> On Thu, 17 Jan 2013 06:35:29 -0800, Mark Carter wrote:
>> 
>>> I thought it would be interesting to try to implement Scheme SRFI 39
>>> (Parameter objects) in Python.
>>> 
>>> The idea is that you define a function that returns a default value.
>>> If you call that function with no arguments, it returns the current
>>> default. If you call it with an argument, it resets the default to the
>>> value passed in. Here's a working implementation:
>> [...]
>>> Can anyone suggest a better implementation?
>> 
>> I don't like the decorator version, because it requires creating a do-
>> nothing function that just gets thrown away. He's my version, a factory
>> function that takes two arguments, the default value and an optional
>> function name, and returns a Param function: [...]
> 
> This, or something like this, is very old:
> 
> sentinel = object()
> class Magic:
>     def __init__(self, value):
>         self.value = value
>     def __call__(self, value=sentinel):
>         if value != sentinel:
>             self.value = value
>         return self.value

There's not really any magic in that :-)

Better to use "if value is not sentinel" rather than != because the 
caller might provide a custom object that compares equal to sentinel.

Also you should name it SENTINEL, or even _SENTINEL, to indicate that it 
is (1) a constant, and (2) a private variable.


> It's not a function, nor a decorator, but it behaves like a function.

A callable, so-called because you can call it :-)

I believe that C++ calls it a "functor", not to be confused with what 
Haskell calls a functor, which is completely different.



-- 
Steven