Sets and Membership

Unit ID: M03-U04 Estimated active time: 20-30 minutes

A set keeps unique values

raw_tags = ["python", "data", "python", "beginner"]
unique_tags = set(raw_tags)
print(unique_tags)

The duplicate "python" is removed. The displayed order may differ because sets are not sequence collections and should not be used when position matters.

Membership is direct

print("python" in unique_tags)
print("advanced" in unique_tags)

The results are True and False.

Add and remove

unique_tags.add("notebook")
unique_tags.discard("advanced")

add() inserts a value. discard() removes a value if present and does nothing if absent. remove() also removes a value but raises KeyError when absent.

Compare sets

required = {"python", "notebook"}
learner_skills = {"python", "notebook", "data"}

print(required.issubset(learner_skills))
print(learner_skills - required)

The subset check is True. The difference contains skills present in learner_skills but not in required.

Useful operations include:

• intersection with &;
• difference with -; and
• symmetric difference with ^.

Empty set reminder

empty_set = set()
empty_dictionary = {}

Use type() if the difference is unclear.

Set items must be hashable

Simple immutable values such as strings, numbers, and tuples of immutable values can normally be set items. A list cannot be a set item because it is mutable.

You do not need the internal hashing details yet. Remember the practical rule: use sets for unique stable values, not nested editable records.

Practice

Create two sets:

course_skills = {"python", "notebook", "data"}
learner_skills = {"python", "spreadsheets"}

Find the common skills, skills still needed for the course, and all unique skills across both sets. Explain why sorting the final values may help when displaying them to a person.

Takeaway

Sets answer uniqueness and membership questions. Do not rely on their order. Next, we will examine how mutable collections can share references and how to copy them deliberately.