CODEWITHSUNDEEP
pythoncounter
Mahesha999
Mahesha999

Reputation: 24811

Does collection's Counter keeps data sorted?

I was reading python collections's Counter. It says following:

>>> from collections import Counter
>>> Counter({'z': 9,'a':4, 'c':2, 'b':8, 'y':2, 'v':2})
Counter({'z': 9, 'b': 8, 'a': 4, 'c': 2, 'y': 2, 'v': 2})

Somehow these printed values are printed in descending order (9 > 8 > 4 > 2). Why is it so? Does Counter store values sorted?

PS: Am on python 3.7.7

Upvotes: 3

Views: 1291

Answers (2)

mozway
mozway

Reputation: 262204

The order depends on the python version.

For python < 3.7, there is no guaranteed order, since python 3.7 the order is that of insertion.

Changed in version 3.7: As a dict subclass, Counter inherited the capability to remember insertion order. Math operations on Counter objects also preserve order. Results are ordered according to when an element is first encountered in the left operand and then by the order encountered in the right operand.

Example on python 3.8 (3.8.10 [GCC 9.4.0]):

from collections import Counter
Counter({'z': 9,'a':4, 'c':2, 'b':8, 'y':2, 'v':2})

Output:

Counter({'z': 9, 'a': 4, 'c': 2, 'b': 8, 'y': 2, 'v': 2})

how to check that Counter doesn't sort by count

As __str__ in Counter return the most_common, it is not a reliable way to check the order.

Convert to dict, the __str__ representation will be faithful.

c = Counter({'z': 9,'a':4, 'c':2, 'b':8, 'y':2, 'v':2})

print(dict(c))
# {'z': 9, 'a': 4, 'c': 2, 'b': 8, 'y': 2, 'v': 2}

Upvotes: 3

0x5453
0x5453

Reputation: 13599

In terms of the data stored in a Counter object: The data is insertion-ordered as of Python 3.7, because Counter is a subclass of the built-in dict. Prior to Python 3.7, there was no guaranteed order of the data.

However, the behavior you are seeing is coming from Counter.__repr__. We can see from the source code that it will first try to display using the Counter.most_common method, which sorts by value in descending order. If that fails because the values are not sortable, it will fall back to the dict representation, which, again, is insertion-ordered.

Upvotes: 9

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