Author: Armin Rigo <[email protected]>
Branch:
Changeset: r75807:112c34045210
Date: 2015-02-10 19:18 +0100
http://bitbucket.org/pypy/pypy/changeset/112c34045210/
Log: Document the result of "is" on NaNs.
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
@@ -300,6 +300,18 @@
Notably missing from the list above are ``str`` and ``unicode``. If your
code relies on comparing strings with ``is``, then it might break in PyPy.
+Note that for floats there "``is``" only one object per "bit pattern"
+of the float. So ``float('nan') is float('nan')`` is true on PyPy,
+but not on CPython because they are two objects; but ``0.0 is -0.0``
+is always False, as the bit patterns are different. As usual,
+``float('nan') == float('nan')`` is always False. When used in
+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`__)
+
+.. __:
https://bitbucket.org/pypy/pypy/issue/1974/different-behaviour-for-collections-of
+
Miscellaneous
-------------
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