Python: List Sorting with Multiple Attributes and Mixed Order

Question

Hi!

I have to sort a python list, with multiple attributes. I can do that in ascending order for ALL attributes easily with

L.sort(key=operator.attrgetter(attribute))....

but the problem is, that I have use mixed configurations for ascending/descending... I have to "imitate" a bit the SQL Order By where you can do something like "name ASC, year DESC". Is there a way to do this easily in python without having to implement a custom compare funtion?

Thans a lot!

Answer 1

You can't, but writing the compare function is easy:

def my_cmp(a, b):
    return cmp(a.foo, b.foo) or cmp(b.bar, a.bar)
L.sort(my_cmp)

Answer 2

A custom function will render your code more readable. If you have many sorting operations and you don't want to create those functions though, you can use lambda's:

L.sort(lambda x, y: cmp(x.name, y.name) or -cmp(x.year, y.year))

Answer 3

If your attributes are numeric, you have this.

def mixed_order( a ):
    return ( a.attribute1, -a.attribute2 )

someList.sort( key=mixed_order )

If your attributes includes strings or other more complex objects, you have some choices.

The .sort() method is stable: you can do multiple passes. This is perhaps the simplest. It's also remarkably fast.

def key1( a ): return a.attribute1
def key2( a ): return a.attribute2

someList.sort( key=key2, reverse=True )
someList.sort( key=key1 )

If this is the only sort, you can define your own special-purpose comparison operators. Minimally, you need __eq__ and __lt__. The other four can be derived from these two by simple logic.