# Sort by multiple keys using different orderings [duplicate]

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### Question :

Sort by multiple keys using different orderings [duplicate]

Possible Duplicate:
How to write Python sort key functions for descending values

In Python 3, it’s pretty easy to sort a list of objects lexicographically using multiple keys. For example:

`items.sort(key = lambda obj: obj.firstname, obj.lastname)`

The `reverse` argument lets you specify whether you want ascending or descending order. But what do you do in the case where you want to sort by multiple keys, but you want to sort using descending order for the first key, and ascending order for the second?

For example, suppose we have an object with two attributes, `points` and `name`, where `points` is an `int` and `name` is a `str`. We want to sort a list of these objects by `points` in descending order (so that the object with the greatest number of points comes first), but for objects with an equal number of `points`, we want to sort these by `name` in alphabetical (ascending) order.

How can this be achieved?

There is no built-in way to handle this. For the general case, you must sort twice: first by the secondary sort, then by the primary sort. As @Mark Ransom mentioned in his comment, in many cases the variables are numeric, and so you can use the negative value to flip the ordering.

If you know the type of the variable you’re trying to sort on and how to work with it, you could also write a key function that returns a decreasing value for increasing keys. See this thread for an example for strings. (Basically, you take the negative of the ASCII numerical value of the characters.)

In Python 2, you could also use a `cmp` function instead of a key, but this will likely make the sort slower. Whether it will make it too slow depends on how big and unsorted the list is. In Python 3, the `cmp` argument is gone, but as @Mark Ransom notes you could use `cmp_to_key`.

``````items.sort(key = lambda obj: (obj.firstname, [(-ord(c) for c in obj.lastname)]))
There is `functools.cmp_to_key` to convert a comparison function into a key compatible with the sorting functions. This was provided for sorts which used a comparison function in Python 2 and needed to be converted to Python 3 which no longer allows them.