How to get count dict of items but maintain the order in which they appear?

Question

For example, I need to count how many times a word appears in a list, not sorted by frequency but with the order in which the words appear, i.e. insertion order.

from collections import Counter

words = ['oranges', 'apples', 'apples', 'bananas', 'kiwis', 'kiwis', 'apples']

c = Counter(words)

print(c)

So instead of: {'apples': 3, 'kiwis': 2, 'bananas': 1, 'oranges': 1}

I'd rather get: {'oranges': 1, 'apples': 3, 'bananas': 1, 'kiwis': 2}

And I don't really need this Counter method, any way that will produce correct result is OK for me.

Answer 1

You can use the recipe that uses collections.Counter and collections.OrderedDict:

from collections import Counter, OrderedDict

class OrderedCounter(Counter, OrderedDict):
    'Counter that remembers the order elements are first encountered'

    def __repr__(self):
        return '%s(%r)' % (self.__class__.__name__, OrderedDict(self))

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

words = ["oranges", "apples", "apples", "bananas", "kiwis", "kiwis", "apples"]
c = OrderedCounter(words)
print(c)
# OrderedCounter(OrderedDict([('oranges', 1), ('apples', 3), ('bananas', 1), ('kiwis', 2)]))

Answer 2

On Python 3.6+, dict will now maintain insertion order.

So you can do:

words = ["oranges", "apples", "apples", "bananas", "kiwis", "kiwis", "apples"]
counter={}
for w in words: counter[w]=counter.get(w, 0)+1
>>> counter
{'oranges': 1, 'apples': 3, 'bananas': 1, 'kiwis': 2}

Unfortunately, the Counter in Python 3.6 and 3.7 does not display the insertion order that it maintains; instead, __repr__ sorts the return by the most to least common.

But you can use the same OrderedDict recipe but just use the Python 3.6+ dict instead:

from collections import Counter

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

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

>>> OrderedCounter(words)
OrderedCounter({'oranges': 1, 'apples': 3, 'bananas': 1, 'kiwis': 2})

Or, since Counter is a subclass of dict that maintains order in Python 3.6+, you can just avoid using Counter's __repr__ by either calling .items() on the counter or turning the counter back into a dict:

>>> c=Counter(words)

This presentation of that Counter is sorted by most common element to least and uses Counters __repr__ method:

>>> c
Counter({'apples': 3, 'kiwis': 2, 'oranges': 1, 'bananas': 1})

This presentation is as encountered, or insertion order:

>>> c.items()
dict_items([('oranges', 1), ('apples', 3), ('bananas', 1), ('kiwis', 2)])

Or,

>>> dict(c)
{'oranges': 1, 'apples': 3, 'bananas': 1, 'kiwis': 2}