[Python-ideas] __missing__ object/keyword

[Arnaud Delobelle]

On 6 Nov 2008, at 20:31, George Sakkis wrote:

> 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 ?

Here's a proof-of-concept decorator version:

from inspect import getargspec

def tell_missing(f):
     argnames = getargspec(f)[0][1:]
     def decorated(*args, **kwargs):
         missing = set(argnames[len(args):]) - set(kwargs)
         return f(missing, *args, **kwargs)
     return decorated

 >>> @tell_missing
... def f(missing, x, y=None, z=None):
...     print "Missing arguments:", ', '.join(missing)
...
 >>> f(1, 2, 3)
Missing arguments:
 >>> f(1, 2)
Missing arguments: z
 >>> f(1)
Missing arguments: y, z

-- 
Arnaud