funcutils: Add partial_ordering(), decorator similar to functools.total_ordering()

boltons/funcutils.py

@@ -98,6 +98,24 @@ def copy_function(orig, copy_dict=True):
     return ret
 
 
+def partial_ordering(cls):
+    """Class decorator. Similar to functools.total_ordering, except it
+    is used to define partial orderings (ie. it is possible that x is niether
+    greater than, equal to or less than y). It assumes the presence
+    of a <= (__le__) and >= (__ge__) method, but nothing else.
+    It will not override any existing additional comparison methods.
+    """
+    def __lt__(self, other): return self <= other and not self >= other
+    def __gt__(self, other): return self >= other and not self <= other
+    def __eq__(self, other): return self >= other and self <= other
+
+    if not hasattr(cls, '__lt__'): cls.__lt__ = __lt__
+    if not hasattr(cls, '__gt__'): cls.__gt__ = __gt__
+    if not hasattr(cls, '__eq__'): cls.__eq__ = __eq__
+
+    return cls
+
+
 class InstancePartial(functools.partial):
     """:class:`functools.partial` is a huge convenience for anyone
     working with Python's great first-class functions. It allows