I discovered today that Python's min and max functions accept a key keyword that allows you to pass a function that returns a value to minimise. This can be used to return elements out of tuples or attributes from objects. For example:

>>> min ((2,1), (3,0), (4,1), key = lambda x:x[1])
(3, 0)

This functionality turns out to be really useful (like when you're merging bounding rectangles). Unfortunately it's not available in Python 2.4, but it is possible to reimplement:

import sys

if sys.version_info[:2] < (2, 5):
        print >> sys.stderr, "INFO: using reimplementations of min/max()"

        def _min_max_mapper (func, args, kwargs):

                if kwargs.has_key ('key'):
                        # reimplement
                        key = kwargs['key']

                        if len (args) == 1 and type (args[0]) is list:
                                values = args[0]
                        else:
                                values = args

                        mappedvalues = [ key (arg) for arg in values ]
                        value = func (mappedvalues)

                        return values[mappedvalues.index (value)]
                else:
                        return func (*args)

        _realmin = min
        _realmax = max

        min = lambda *a, **b: _min_max_mapper (_realmin, a, b)
        max = lambda *a, **b: _min_max_mapper (_realmax, a, b)

Importing this will implement Python 2.5 min/max semantics in earlier versions on Python.