When working with set or frozenset container, one of the most common operations we can perform is a merge. To do this, we can return the union of two sets using the .union() method or | operator. Doing so will return a new set or frozenset containing all elements from both sets without duplicates.

Take a look at the Venn diagram representing a union of set A and set B:

Python Set Union

Notice the resulting set contains all the elements in both set A and set B as well as elements they have in common (minus the duplicates). In this case we are only looking at merging two sets but it’s also common to perform the operation on as many as we need!

Let’s look at two examples of creating a union:

  1. Using union():

    # Given a set and frozenset of song tags for two python related hits prepare_to_py = {'rock', 'heavy metal', 'electric guitar', 'synth'} py_and_dry = frozenset({'classic', 'rock', 'electric guitar', 'rock and roll'}) # Get the union using the .union() method combined_tags = prepare_to_py.union(py_and_dry) print(combined_tags)

    Would output:

    {'electric guitar', 'classic', 'heavy metal', 'rock and roll', 'rock', 'synth'}
  2. Using |:

    # Get the union using the | operator frozen_combined_tags = py_and_dry | prepare_to_py print(frozen_combined_tags)

    Would output:

    frozenset({'electric guitar', 'rock and roll', 'rock', 'synth', 'heavy metal', 'classic'})

Note that the return value in both methods takes the form of the left operand. In the first example since prepare_to_py() called the union() function, so the result was a regular set. In the second example, since py_and_dry was the left operand, the end result was a frozenset.



To improve the logic for adding user tags to songs in the app, we can use the union of tag sets! Our team has provided us two dictionaries.

The first dictionary (song_data) contains song data including tags from the original artists, while the second dictionary (user_tag_data) includes tags that have been added by users. Let’s attempt to merge the tag sets together so we have a full collection of tags.

First, create an empty dictionary called new_song_data which will hold the merged tag data.


Our goal now is to consolidate the tags into one dictionary for each category. To accomplish this we need to:

  • Loop over song_data.items() (all the items in song_data)

    • On each iteration of the loop, create a set for each category of tags. This will require creating two new sets, one for song_data and one for user_tag_data.

    • In addition to creating the sets on each iteration, create a new key inside of new_song_data for each category and set the value to be a union of the two new sets.

Print new_song_data to see the result!

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