closes bpo-28955: Clarified comparisons between NaN and number in reference documentation (GH-5982)
Co-authored-by: Benjamin Peterson <benjamin@python.org>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 33f7575..f4b1618 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -1336,12 +1336,11 @@
involved, they compare mathematically (algorithmically) correct without loss
of precision.
- The not-a-number values :const:`float('NaN')` and :const:`Decimal('NaN')`
- are special. They are identical to themselves (``x is x`` is true) but
- are not equal to themselves (``x == x`` is false). Additionally,
- comparing any number to a not-a-number value
- will return ``False``. For example, both ``3 < float('NaN')`` and
- ``float('NaN') < 3`` will return ``False``.
+ The not-a-number values ``float('NaN')`` and ``decimal.Decimal('NaN')`` are
+ special. Any ordered comparison of a number to a not-a-number value is false.
+ A counter-intuitive implication is that not-a-number values are not equal to
+ themselves. For example, if ``x = float('NaN')``, ``3 < x``, ``x < 3``, ``x
+ == x``, ``x != x`` are all false. This behavior is compliant with IEEE 754.
* Binary sequences (instances of :class:`bytes` or :class:`bytearray`) can be
compared within and across their types. They compare lexicographically using