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| -rw-r--r-- | Doc/reference/expressions.rst | 26 |
1 files changed, 7 insertions, 19 deletions
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst index 432327a..ee13c5f 100644 --- a/Doc/reference/expressions.rst +++ b/Doc/reference/expressions.rst @@ -1425,6 +1425,10 @@ built-in types. 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. +* ``None`` and ``NotImplemented`` are singletons. :PEP:`8` advises that + comparisons for singletons should always be done with ``is`` or ``is not``, + never the equality operators. + * Binary sequences (instances of :class:`bytes` or :class:`bytearray`) can be compared within and across their types. They compare lexicographically using the numeric values of their elements. @@ -1442,25 +1446,9 @@ built-in types. :exc:`TypeError`. Sequences compare lexicographically using comparison of corresponding - elements, whereby reflexivity of the elements is enforced. - - In enforcing reflexivity of elements, the comparison of collections assumes - that for a collection element ``x``, ``x == x`` is always true. Based on - that assumption, element identity is compared first, and element comparison - is performed only for distinct elements. This approach yields the same - result as a strict element comparison would, if the compared elements are - reflexive. For non-reflexive elements, the result is different than for - strict element comparison, and may be surprising: The non-reflexive - not-a-number values for example result in the following comparison behavior - when used in a list:: - - >>> nan = float('NaN') - >>> nan is nan - True - >>> nan == nan - False <-- the defined non-reflexive behavior of NaN - >>> [nan] == [nan] - True <-- list enforces reflexivity and tests identity first + elements. The built-in containers typically assume identical objects are + equal to themselves. That lets them bypass equality tests for identical + objects to improve performance and to maintain their internal invariants. Lexicographical comparison between built-in collections works as follows: |