[Python-Dev] PyObject_RichCompareBool identity shortcut (original) (raw)

Glenn Linderman v+python at g.nevcal.com
Wed Apr 27 20:41:15 CEST 2011

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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