Cool way to sort a dict by key or value

A simple way to sort a dict is to use iteritems() with a lamdba function, like so:

somedict = {...}
[ x[0] for x in sorted(somedict.iteritems(), lambda x, y: cmp(x[1], y[1])) ]

The cooler (and faster) way is to use itemgetter from the operator module, with the key parameter to sorted().

To sort a dict by keys, use:

import operator
somedict = {...}
[k for k, v in sorted(d.iteritems(), key=itemgetter(0))]

To sort a dict by values, instead use the line:

import operator
somedict = {...}
[k for k, v in sorted(d.iteritems(), key=itemgetter(1))]

Remember how itemgetter works: it is identical to the key lookup operator. In other words, in the code below,

somedict = {'a': 1}
operator.itemgetter('a')(somedict)
somedict['a']

the last two lines do exactly the same thing. This is a neat trick to avoid a lambda call, since subroutine calls are relatively expensive.