The other day I was surprised to learn this:
>>> nan = float('nan')
>>> nan == nan
False
>>> [nan] == [nan]
True # also True in tuples, dicts, etc.
# also:
>>> l = [nan]
>>> nan in l
True
>>> l.index(nan)
0
>>> l[0] == nan
False
The identity test is not in container comparators, but in
PyObject_RichCompareBool:
/* Quick result when objects are the same.
Guarantees that identity implies equality. */
if (v == w) {
if (op == Py_EQ)
return 1;
else if (op == Py_NE)
return 0;
}
The guarantee referred to in the comment is not only (AFAICT)
undocumented, but contradicts the documentation, which states that the
result should be the "equivalent of o1 op o2".
Calling PyObject_RichCompareBool is inconsistent with calling
PyObject_RichCompare and converting its result to bool manually,
something that wrappers (C++) and generators (cython) might reasonably
want to do themselves, for various reasons.
If this is considered a bug, I can open an issue.
Hrvoje
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