Passing a keyword argument to a subclass of the built-in class "set" causes a TypeError in Python 2.4.2 and newer but not in 2.4.0. Example:

class Test(set):
    def __init__(self, a=0):
        self.add(a)

t = Test(a=0)

In Python 2.4.0 it works as expected whereas in 2.4.2 and 2.4.4, it yields:

Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: set() does not take keyword arguments

I don't pass a keyword argument to set.__init__() but to its subclass!

2.4 is no longer supported.  This has been fixed in 2.5.  I don't have the original bug report #.  You can find the fix in SVN if you would like to build your own patched version.