python 具有多个属性和混合顺序的列表排序
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List sorting with multiple attributes and mixed order
提问by Bernhard Vallant
I have to sort a list with multiple attributes. I can do that in ascending order for ALL attributes easily with
我必须对具有多个属性的列表进行排序。我可以轻松地按升序为所有属性执行此操作
L.sort(key=operator.attrgetter(attribute))....
but the problem is, that I have to use mixed configurations for ascending/descending... I have to "imitate" a bit the SQL Order Bywhere 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 function?
但问题是，我必须使用混合配置进行升序/降序......我必须“模仿”一些 SQL Order By，你可以在其中执行类似name ASC, year DESC. 有没有办法在 Python 中轻松做到这一点，而无需实现自定义比较函数？
回答by S.Lott
If your attributes are numeric, you have this.
如果你的属性是数字，你有这个。
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.
如果您的属性包括字符串或其他更复杂的对象，您有一些选择。
The .sort()method is stable: you can do multiple passes. This is perhaps the simplest. It's also remarkably fast.
该.sort()方法是稳定的：您可以进行多次传递。这也许是最简单的。它也非常快。
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.
如果这是唯一的排序，您可以定义自己的专用比较运算符。至少，您需要__eq__和__lt__。其他四个可以通过简单的逻辑从这两个导出。
回答by RedGlyph
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:
自定义函数将使您的代码更具可读性。如果您有很多排序操作并且不想创建这些函数，则可以使用 lambda 的：
L.sort(lambda x, y: cmp(x.name, y.name) or -cmp(x.year, y.year))
回答by Luká? Lalinsky
You can't, but writing the compare function is easy:
你不能，但编写比较函数很容易：
def my_cmp(a, b):
return cmp(a.foo, b.foo) or cmp(b.bar, a.bar)
L.sort(my_cmp)
