What Is Bag Data Structure in Python?


Heather Bennett

A bag data structure in Python is a collection that allows for the storage of items without any specific order. It is also known as a multiset.

Unlike other data structures like lists or arrays, bags do not enforce uniqueness of elements. This means that items can appear multiple times within a bag.

Creating a Bag in Python

To create a bag in Python, you can use the built-in collections.Counter() class from the collections module. This class provides an easy way to count the occurrences of elements in an iterable.


from collections import Counter

# Create a new bag
bag = Counter()

# Add elements to the bag
bag['apple'] += 1
bag['banana'] += 2
bag['orange'] += 3


The output of this code will be:

Counter({'orange': 3, 'banana': 2, 'apple': 1})

Accessing Elements in a Bag

To access elements in a bag, you can use the same syntax as accessing elements in a dictionary. The elements are stored as keys, and their counts are stored as values.

# Accessing elements in the bag
print(bag['banana'])  # Output: 2
print(bag['grape'])   # Output: 0 (element not present)

Iterating Over a Bag

You can iterate over the elements in a bag using a for loop. The loop will iterate over each unique element in the bag, considering their counts.

# Iterating over the bag
for item, count in bag.items():
    print(item, count)

apple 1
banana 2
orange 3

Modifying a Bag

To modify the contents of a bag, you can use various methods provided by the Counter() class. Some of these methods include:

  • update(): Adds elements from another iterable or mapping to the bag.
  • subtract(): Removes elements from another iterable or mapping from the bag.
  • clear(): Removes all elements from the bag.

# Modifying the bag
bag.update({'banana': 1, 'grape': 5})

bag.subtract({'orange': 2})


Counter({'orange': 3, 'grape': 5, 'banana': 3, 'apple': 1})
Counter({'grape': 5, 'banana': 2, 'apple': 1})


A bag data structure in Python allows for the storage of items without any specific order and allows duplicates. It can be created using the collections.Counter() class and provides methods for accessing, iterating over, and modifying its contents. Bags are useful in scenarios where the frequency of elements is important, such as counting word occurrences in a text document or analyzing customer purchase history.

By understanding and utilizing the bag data structure, you can effectively handle situations that require storing and manipulating collections of items with repetitions.

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