[Python-ideas] __missing__ object/keyword

Michael fuzzyman at gmail.com
Sat Nov 8 12:36:29 CET 2008

This seems to be a solution in search of a problem. If you handle  
sentinel default values early in a function then they are rarely  

Michael Foord


On 8 Nov 2008, at 05:37, "Bruce Leban" <bruce at leapyear.org> wrote:

> The problem with using a value like None as some have proposed is  
> that None may be a valid parameter value. Thus the only safe value  
> is a special value like a throwaway object. There are numerous  
> problems with this including that it's not safe: I might not check  
> for the missing value in some place and end up passing the throwaway  
> object to another function which then passes it as one of the  
> optional values.
> There *is* one guaranteed way of ensuring that I can't use a  
> variable's value: leaving it unbound. That is, to support this, we  
> would add a new syntax something like what George proposed with  
> slightly different semantics:
>   def foo(x, y=__unbound__):
> where if y is omitted from the call, then y is unbound. This is  
> slightly different than what would happen if y were left out of the  
> parameter list as in that case, y could reference a global of the  
> same name. In this case, y can only reference an unbound local.
> No other changes are required to use this. If I don't check whether  
> or not y is bound, then I'll get a NameError when I try to use it,  
> just as with any other unbound variable.
> If this proposal were to be seriously considered, there are of  
> course alternative syntaxes that could be considered, like using  
> missing, __missing__ or def foo(x, ?y) but that's getting ahead.
> I agree it would be nice to have a way to check whether a variable  
> is unbound without writing a multi-line try/except, but I think that  
> can be discussed as a different issue.
> --- Bruce
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