Python Cookbook

Sorting a List of Objects by an Attribute of the Objects

Credit: Yakov Markovitch, Nick Perkins

Problem

You have a list of objects that you need to sort according to one attribute of each object, as rapidly and portably as possible.

Solution

In this case, the obvious approach is concise, but quite slow:

def sort_by_attr_slow(seq, attr):
    def cmp_by_attr(x, y, attr=attr):
        return cmp(getattr(x, attr), getattr(y, attr))
    seq.sort(cmp_by_attr)

There is a faster way, with DSU:

def sort_by_attr(seq, attr):
    import operator

    intermed = map(None, map(getattr, seq, (attr,)*len(seq)),
        xrange(len(seq)), seq)
    intermed.sort(  )
    return map(operator.getitem, intermed, (-1,)*len(intermed))

def sort_by_attr_inplace(lst, attr):
    lst[:] = sort_by_attr(lst, attr)

Discussion

Sorting a list of objects by an attribute of each object is best done using the DSU idiom. Since this recipe uses only built-ins and doesn’t use explicit looping, it is quite fast. Moreover, the recipe doesn’t use any Python 2.0-specific features (such as zip or list comprehensions), so it can be used for Python 1.5.2 as well.

List comprehensions are neater, but this recipe demonstrates that you can do without them, if and when you desperately need portability to stone-age Python installations. Of course, the correct use of map can be tricky. Here, for example, when we build the auxiliary list intermed, we need to call the built-in function getattr on each item of sequence seq using the same string, attr, as the second argument in each case. ...