Python: sort an array of dictionaries with custom comparator?

Question

I have the following Python array of dictionaries:

myarr = [ { \'name\': \'Richard\', \'rank\': 1 },
{ \'name\': \'Reuben\', \'rank\': 4 },
{ \'name\': \'Reece\'         


        

Answer 1

You're myarr binding here doesn't look like valid Python code (and doesn't execute in my interpreter session.

Rendering that into:

myarr = {
    'Richard': 1,
    'Reuben': 4,
    'Reece': 0,
    'Rohan': 3,
    'Ralph': 2,
    'Raphael': 0,
    'Robin': 0 }

Gives me something on which I could base an answer.

The recommended way to do custom sorting in Python is to use the DSU (decorate, sort, undecorate) pattern. If you want to sort a dictionary by values then that looks something like:

keys_sorted_by_val = [ x[1] for x in sorted([(v,k) for k,v in myarr.items()])]

... where (v,k) for k,v in myarr.items() is the expression to decorate; sorted() is, obviously, the sort and the outer x[1] for x in ... is the final undecorate step.

Obviously this might seem like a sufficiently common requirement that one might which to wrap this up in a function:

def dict_by_values(d):
    return [ x[1] for x in sorted([(v,k) for k,v in d.items()])]

If you had a collection of object instances that you want to sort by some attribute you can use something like this:

def sort_by_attr(attr, coll):
    results = list()
    for each in coll:
        assert hasattr(each, attr)
        results.append((getattr(each, attr), each))
    results.sort()
    return [x[1] for x in results]

So if we created a class representing your name/rank data like this:

class NameRanking(object):
    def __init__(self, name, rank):
        self.name = name
        self.rank = rank
    def __repr__(self):
        return "%s: %s, %s" %(self.__class__, self.name, self.rank)

... and instantiate a list of those using myarr:

name_rankings = [ NameRanking(k, v) for k,v in myarr.items() ]

... then we could get a sorted copy of that using:

names_rankings_by_rank = sort_by_attr('rank', name_rankings)

(Yes the assert isn't a good idea here; that's where you would put in your own exception handling or throwing code as appropriate to your application).