News item for rich comparisons.
(I'm going to check in some more uses of rich comparisons, but the
basic feature should be in place now.)
diff --git a/Misc/NEWS b/Misc/NEWS
index 40e22eb..15f4513 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -3,6 +3,43 @@
Core language, builtins, and interpreter
+- The comparison operators support "rich comparison overloading" (PEP
+ 207). C extension types can provide a rich comparison function in
+ the new tp_richcompare slot in the type object. The cmp() function
+ and the C function PyObject_Compare() first try the new rich
+ comparison operators before trying the old 3-way comparison. There
+ is also a new C API PyObject_RichCompare() (which also falls back on
+ the old 3-way comparison, but does not constrain the outcome of the
+ rich comparison to a Boolean result).
+
+ The rich comparison function takes two objects (at least one of
+ which is guaranteed to have the type that provided the function) and
+ an integer indicating the opcode, which can be Py_LT, Py_LE, Py_EQ,
+ Py_NE, Py_GT, Py_GE (for <, <=, ==, !=, >, >=), and returns a Python
+ object, which may be NotImplemented (in which case the tp_compare
+ slot function is used as a fallback, if defined).
+
+ Classes can overload individual comparison operators by defining one
+ or more of the methods__lt__, __le__, __eq__, __ne__, __gt__,
+ __ge__. There are no explicit "reversed argument" versions of
+ these; instead, __lt__ and __gt__ are each other's reverse, likewise
+ for__le__ and __ge__; __eq__ and __ne__ are their own reverse
+ (similar at the C level). No other implications are made; in
+ particular, Python does not assume that == is the inverse of !=, or
+ that < is the inverse of >=. This makes it possible to define types
+ with partial orderings.
+
+ Classes or types that want to implement (in)equality tests but not
+ the ordering operators (i.e. unordered types) should implement ==
+ and !=, and raise an error for the ordering operators.
+
+ It is possible to define types whose comparison results are not
+ Boolean; e.g. a matrix type might want to return a matrix of bits
+ for A < B, giving elementwise comparisons. Such types should ensure
+ that any interpretation of their value in a Boolean context raises
+ an exception, e.g. by defining __nonzero__ (or the tp_nonzero slot
+ at the C level) to always raise an exception.
+
- Functions and methods now support getting and setting arbitrarily
named attributes (PEP 232). Functions have a new __dict__
(a.k.a. func_dict) which hold the function attributes. Methods get
@@ -70,6 +107,12 @@
supported -- instead of calling __rcmp__, __cmp__ is called with
reversed arguments.
+- In connection with the coercion changes, a new built-in singleton
+ object, NotImplemented is defined. This can be returned for
+ operations that wish to indicate they are not implemented for a
+ particular combination of arguments. From C, this is
+ Py_NotImplemented.
+
- The interpreter accepts now bytecode files on the command line even
if they do not have a .pyc or .pyo extension. On Linux, after executing