If you want to know if an object is sortable, you must check if it implements the necessary methods of comparison.
In Python 2.X there were two different ways to implement those methods:
cmp method (equivalent of compareTo in Java per example)
__cmp__(self, other)
: returns >0, 0 or <0 wether self is more, equal or less than otherrich comparison methods
__lt__, __gt__, __eq__, __le__, __ge__, __ne__
The sort() functions call this method to make the necessary comparisons between instances (actually sort only needs the
__lt__
or__gt__
methods but it's recommended to implement all of them)
In Python 3.X the __cmp__
was removed in favor of the rich comparison methods as having more than one way to do the same thing is really against Python's "laws".
So, you basically need a function that check if these methods are implemented by a class:
# Python 2.X
def is_sortable(obj):
return hasattr(obj, "__cmp__") or \
hasattr(obj, "__lt__") or \
hasattr(obj, "__gt__")
# Python 3.X
def is_sortable(obj):
cls = obj.__class__
return cls.__lt__ != object.__lt__ or \
cls.__gt__ != object.__gt__
Different functions are needed for Python 2 and 3 because a lot of other things also change about unbound methods, method-wrappers and other internal things in Python 3.
Read this links you want better understanding of the sortable objects in Python:
http://python3porting.com/problems.html#unorderable-types-cmp-and-cmp
http://docs.python.org/2/howto/sorting.html#the-old-way-using-the-cmp-parameter
PS: this was a complete re-edit of my first answer, but it was needed as I investigated the problem better and had a cleaner idea about it :)