More about the __cmp__ method in Python

I just stumbled upon this little something about overloading the __cmp__ method in a Python class. If you are not careful, you will end up having a class whose instances can not be compared to None (as I always do to see if some optional parameter has been passed to a method or function). Lets take a small example:

class Foo(object):

def __init__(self, x):

    self._x = x

def __cmp__(self, other):

    if type(other) == Foo:

        return cmp(self._x, other._x)

    else:

        raise TypeError("Foo cannot be compared to %s"%str(type(other))) f1 = Foo(1)

f2 = Foo(2)

print 'f1 is smaller than f2:', f1 < f2 # True

print 'f1 is larger than f2:', f1 > f2  # False

print 'f1 is equal to f2:', f1 == f2    # False

print 'f1 is None:', f1 == None         # Crash!

This code crashes in the final statement, because the comparison to None calls the __cmp__ method, and it has no way of comparing to None. This could of course be fixed by adding "if other == None: return -1" but the right way to fix this would be by overloading another method called __eq__ that is used to test for object equality.

class Foo(object):

def __init__(self, x):

    self._x = x

def __eq__(self, other):

    if type(other) == Foo:

        return cmp(self, other) == 0

    else:

        return False

def __cmp__(self, other):

    if type(other) == Foo:

        return cmp(self._x, other._x)

     else:

         raise TypeError("Foo cannot be compared to %s"%str(type(other)))



f1 = Foo(1)

f2 = Foo(2)

print 'f1 is smaller than f2:', f1 < f2 # True print 'f1 is larger than f2:', f1 > f2  # False

print 'f1 is equal to f2:', f1 == f2    # False

print 'f1 is None:', f1 == None         # False

Now the comparison to None (or in fact to any object that is not of type Foo) is caught by the __eq__ method, and comparisons to other type Foo objects are correctly forwarded to the __cmp__ method.