Chris Curvey wrote:
> I must be having a brain cramp. Given a list of objects, how can I
> sort the list on one attribute in descending order, then sort within
> each group in ascending order on another attribute.
>
> For example:
>
> class Foo:
> def __init__(self, a, b, c):
> self.a = a
> self.b = b
> self.c = c
>
>
> I can do "allmyfoos.sort(operator.attrgetter('a','b'))" to sort
> ascending on both attributes, but how could i sort by "a, descending,
> then b, ascending)?"
In the general case you have to sort twice:
>>> from collections import namedtuple
>>> Foo = namedtuple("Foo", "a b")
>>> items = [Foo(x, y) for x in range(3) for y in range(3)]
>>> def show(items):
... for item in items: print item
...
>>> show(items)
Foo(a=0, b=0)
Foo(a=0, b=1)
Foo(a=0, b=2)
Foo(a=1, b=0)
Foo(a=1, b=1)
Foo(a=1, b=2)
Foo(a=2, b=0)
Foo(a=2, b=1)
Foo(a=2, b=2)
>>> from operator import attrgetter
>>> items.sort(key=attrgetter("b"))
>>> items.sort(key=attrgetter("a"), reverse=True)
>>> show(items)
Foo(a=2, b=0)
Foo(a=2, b=1)
Foo(a=2, b=2)
Foo(a=1, b=0)
Foo(a=1, b=1)
Foo(a=1, b=2)
Foo(a=0, b=0)
Foo(a=0, b=1)
Foo(a=0, b=2)
This is guaranteed to work because list.sort() is "stable", i. e. it doesn't
alter the order of items with equal keys.
Peter
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