[Python-ideas] __missing__ object/keyword

[Bruce Leban]

On Sat, Nov 8, 2008 at 12:19 PM, Terry Reedy <tjreedy at udel.edu> wrote:

> Bruce Leban wrote:
>
>>
>>  There *is* one guaranteed way of ensuring that I can't use a variable's
>> value: leaving it unbound.
>>
>
> This Perl, etc. concept does not fit Python's name-object model.  A Python
> mappings and sequences cannot have a key or index without a value.  To
> propose otherwise is a major conceptual change.
>
> tjr
>

What I would expect (as a programmer) is that when the parameter is omitted,
the variable is simply not bound and therefore it's omitted from the
mapping, not added to the mapping with a special magic value meaning
unbound.

However, there is an implementation complication due to the way variable
scoping works in Python: if a variable is left unbound in the function
locals, then references to that variable name would pick up the global if it
exists which would definitely be a surprising result. So I agree that there
would definitely be some complexities to implement this.

--- Bruce
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