Recipe5.1.Sorting a Dictionary

Recipe 5.1. Sorting a Dictionary

Credit: Alex Martelli

Problem

You want to sort a dictionary. This probably means that you want to sort the keys and then get the values in that same sorted order.

Solution

The simplest approach is exactly the one expressed by the problem statement: sort the keys, then pick the corresponding values:

def sortedDictValues(adict):     keys = adict.keys( )     keys.sort( )     return [adict[key] for key in keys]

Discussion

The concept of sorting applies only to a collection that has an orderin other words, a sequence. A mapping, such as a dictionary, has no order, so it cannot be sorted. And yet, "How do I sort a dictionary?" is a frequent, though literally meaningless, question on the Python lists. More often than not, the question is in fact about sorting some sequence composed of keys and/or values from the dictionary.

As for the implementation, while one could think of more sophisticated approaches, it turns out (not unusually, for Python) that the one shown in the solution, the simplest one, is also essentially the fastest one. A further slight increase in speed, about 20%, can be squeezed out in Python 2.3 by replacing the list comprehension with a map call in the return statement at the end of the function. For example:

    return map(adict.get, keys)

Python 2.4, however, is already measurably faster than Python 2.3 with the version in the "Solution" and gains nothing from this further step. Other variants, such as using adict._ _getitem_ _ instead of adict.get, offer no further increase in speed, or they even slow performance down a little, in both Python 2.3 and 2.4.

Recipe5.1.Sorting a Dictionary