On 4/27/2011 8:31 AM, Nick Coghlan wrote:
What that means is that "correct" implementations of methods like
__contains__, __eq__, __ne__, index() and count() on containers should
be using "x is y or x == y" to enforce reflexivity, but most such code
does not (e.g. our own collections.abc.Sequence implementation gets
those of these that it implements wrong, and hence Sequence based
containers will handle NaN in a way that differs from the builtin
containers)
+1 to everything Nick said.
One issue that I don't fully understand: I know there is only one
instance of None in Python, but I'm not sure where to discover whether
there is only a single, or whether there can be multiple, instances of
NaN or Inf. The IEEE 754 spec is clear that there are multiple bit
sequences that can be used to represent these, so I would hope that
there can be, in fact, more than one value containing NaN (and Inf).
This would properly imply that a collection should correctly handle the
case of storing multiple, different items using different NaN (and Inf)
instances. A dict, for example, should be able to hold hundreds of
items with the index value of NaN.
The distinction between "is" and "==" would permit proper operation, and
I believe that Python's "rebinding" of names to values rather than the
copying of values to variables makes such a distinction possible to use
in a correct manner.
Can someone confirm or explain this issue?
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