[pypy-commit] pypy default: test and document the cmp(nan, nan) == 0 behaviour

[cfbolz]

Author: Carl Friedrich Bolz-Tereick <cfbolz at gmx.de>
Branch: 
Changeset: r93361:96cf224f204a
Date: 2017-12-11 13:34 +0100
http://bitbucket.org/pypy/pypy/changeset/96cf224f204a/

Log:	test and document the cmp(nan, nan) == 0 behaviour

diff --git a/pypy/doc/cpython_differences.rst b/pypy/doc/cpython_differences.rst
--- a/pypy/doc/cpython_differences.rst
+++ b/pypy/doc/cpython_differences.rst
@@ -355,7 +355,11 @@
 containers (as list items or in sets for example), the exact rule of
 equality used is "``if x is y or x == y``" (on both CPython and PyPy);
 as a consequence, because all ``nans`` are identical in PyPy, you
-cannot have several of them in a set, unlike in CPython.  (Issue `#1974`__)
+cannot have several of them in a set, unlike in CPython.  (Issue `#1974`__).
+Another consequence is that ``cmp(float('nan'), float('nan')) == 0``, because
+``cmp`` checks with ``is`` first whether the arguments are identical (there is
+no good value to return from this call to ``cmp``, because ``cmp`` pretends
+that there is a total order on floats, but that is wrong for NaNs).
 
 .. __: https://bitbucket.org/pypy/pypy/issue/1974/different-behaviour-for-collections-of
 
diff --git a/pypy/module/__builtin__/test/test_builtin.py b/pypy/module/__builtin__/test/test_builtin.py
--- a/pypy/module/__builtin__/test/test_builtin.py
+++ b/pypy/module/__builtin__/test/test_builtin.py
@@ -404,6 +404,7 @@
 
 
     def test_cmp(self):
+        assert cmp(float('nan'), float('nan')) == 0
         assert cmp(9,9) == 0
         assert cmp(0,9) < 0
         assert cmp(9,0) > 0