1. If you run sort on a list without any parameters, it will be sorted into its "natural" order.  The natural sort order is defined by the __cmp__ method on the classe(s) of objec ts in the list.

2. If you run sort on a list and provide a method name as parameter, that method is used as the comparator.  It takes two incoming parameters, and return -1 0 or +1 to indicate which of the two incoming parameters has the higher sort value (or 0 if they have the same value as far as sort is concerned).

Here's an example.  Our postman has mail to deliver to a number of houses and (in British fashion) wants to go up one side of the street - the even numbers, then back through the odd numbers on the other side.

Code:

values = [1,2,5,12,14,16,10,19,35,38,41,22] values.sort() print values def postman(one, two):        if one%2 == 0 and two%2 != 0: return -1        if one%2 != 0 and two%2 == 0: return 1        if not one%2: return cmp(one, two)        return cmp(two,one) values.sort(postman) print values

And the results:

[1, 2, 5, 10, 12, 14, 16, 19, 22, 35, 38, 41]
[2, 10, 12, 14, 16, 22, 38, 41, 35, 19, 5, 1]

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