[Python-ideas] __missing__ object/keyword

[Mathias Panzenböck]

Why not doing this?

def foo(x, y=None):
   if y is None:
     do_this
   else:
     do_that

George Sakkis schrieb:
> Several times I find myself using the following idiom:
> 
> _Missing = object()   # sentinel
> 
> def foo(x, y=_Missing):
>    if y is _Missing:
>      do_this
>    else:
>      do_that
> 
> The reason for using a "private" sentinel is that any python object,
> including None, might be a valid argument. Another option is using
> *args or **kwds instead of y but that obfuscates unnecessarily the
> function signature.
> 
> It would be nice if a new object or keyword, say __missing__, was
> introduced as a canonical way to address this common scenario.
> Specifically, the only valid usages of __missing__ would be:
> 1. As a default argument in a callable.
> 2. In identity tests: <var> is __missing__
> Anything else would raise either a SyntaxError (e.g. `x =
> __missing__`) or a RuntimeError/TypeError (e.g. `x = y` if y is
> __missing__). Only the interpreter could assign __missing__ to a name
> when binding objects to formal parameters.
> 
> If this was to be accepted, a further generalization could be to allow
> the `var is __missing__` expression even if `var` is not a formal
> parameter. This would be equivalent to:
> 
> try: var
> except NameError: expr = True
> else: expr = False
> 
> Thoughts ?
> 
> George
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