How to get the python Counter output ordered by order of inputs?

Question

I have been working on getting the count(frequency) and then making the graph representation of it.

I am using Counter class from collections using python. I want the output of the Counter in the order of the first come object.

for example:

offset=['a','b','c','a','b','b','b','c','c','c','c','c']
counts = Counter(offset)
print counts

output is:

Counter({'c': 6, 'b': 4, 'a': 2})

As I want the output to be as follows, in the order of the first come object:

Counter({'a': 2, 'b': 4, 'c': 6})

Is that possible?

Thanks

Answer 1

OrderedCounter by (2 lines long) multiple inheritance

Following wonderful speech Super considered super by Raymond Hettinger we can use multiple inheritance:

from collections import OrderedDict, Counter

class OrderedCounter(Counter, OrderedDict):
    pass

and use it:

>>> counter = OrderedCounter("abracadabra")
>>> for key, value in counter.items():
...    print key, value
a 5
b 2
r 2
c 1
d 1

This is all you need for your task.

Fixing a slightly confusing representation

Let us do a bit more tests and see, what "result" we get:

>>> counter = OrderedCounter("cbaaa")
>>> counter
OrderedCounter({'a': 3, 'c': 1, 'b': 1})

Oh - this looks wrong, expected order is "c", "b", "a". Let us test it printing the keys and values:

>>> for key, value in counter.items():
...    print key, value
c 1
b 1
a 3

This looks correct (and it is exactly what really counts for using in your code).

It turns out, that the class we have created is just producing a bit confusing representation.

This can be fixed:

class OrderedCounter(Counter, OrderedDict):
    def __repr__(self):
        return "%s(%r)" % (self.__class__.__name__, OrderedDict(self))

and when used:

>>> counter = OrderedCounter("cbaaa")
>>> counter
OrderedCounter({'c': 1, 'b': 1, 'a': 1})

Full version from the Raymond Hettinger speech (added pickling)

Full version provided in the speech is adding one more method __reduce__ which allows pickling the object properly.

from collections import OrderedDict, Counter

class OrderedCounter(Counter, OrderedDict):
    """Counter that remembers the order elements are first seen"""
    def __repr__(self):
        return "%s(%r)" % (self.__class__.__name__, OrderedDict(self))

    def __reduce__(self):
        return self.__class__, (OrderedDict(self),)

Anyway, in most cases you will manage with the simplest version of the OrderedCounter class.