Python Sets (set): Removing Duplicates and Basic Creation

In addition to lists and tuples, Python has a collection type called Set (set) for handling groups of data. Based on the mathematical concept of sets, the set type has two very important characteristics:

1. No Duplicate Elements: If you try to add the same element multiple times, it is automatically combined into one.
2. Unordered: It does not maintain a specific order.

Sets are used to efficiently remove duplicate elements from a list, perform high-speed existence checks, or perform set operations (like union and intersection).

This article explains the basic ways to create sets.

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1. Creating with {} (Curly Braces)

The basic way to create a set is to enclose elements in {} (curly braces), separated by commas.

# Create a set with curly braces
# Note that "web" is duplicated
tags = {"python", "django", "web", "api", "web"}

print(tags)
print(f"Type: {type(tags)}")

Output:

{'api', 'python', 'django', 'web'}
Type: <class 'set'>

(Note: The order of elements in the output may vary depending on the environment.)

The duplicate “web” was automatically removed, and the set was created with only unique elements.

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2. Creating with set() Constructor (Converting from List)

A very common use case for sets is removing duplicate elements from a list. By passing an iterable (like a list) to the set() constructor (function), it converts it into a set with duplicates removed.

# List of user IDs containing duplicates
user_id_list = ["u001", "u002", "u001", "u003", "u002", "u001"]
print(f"Original List: {user_id_list}")

# Convert list to set using set() to remove duplicates
unique_user_ids = set(user_id_list)
print(f"After Deduplication (set): {unique_user_ids}")

Output:

Original List: ['u001', 'u002', 'u001', 'u003', 'u002', 'u001']
After Deduplication (set): {'u002', 'u003', 'u001'}

If you want to treat it as a list again after removing duplicates, you can convert it back using list(set(user_id_list)).

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3. How to Create an Empty Set (Important)

This is the most important point to remember when creating sets. If you try to create an empty set using only {} (curly braces), Python interprets it as an Empty Dictionary (dict).

# This becomes an empty "Dictionary"
empty_wrong = {}
print(f"Type of {{}}: {type(empty_wrong)}")

Output:

Type of {}: <class 'dict'>

To create an empty set, you must call the set() constructor without arguments.

# The correct way to create an empty set
empty_correct = set()
print(f"Type of set(): {type(empty_correct)}")

Output:

Type of set(): <class 'set'>

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Summary

• set is a data type that does not allow duplicates and is unordered.
• You can create one using {} like {"a", "b", "c"}.
• Using set(list) allows you to easily remove duplicate elements from a list.
• To create an empty set, use set(), not {}.