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Felipe Cezar
Felipe Cezar

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Python {} Explained: When Is It a Set and When Is It a Dictionary?

#python #datascience

In Python, curly braces {} can represent two completely different data structures:

  • A set
  • A dictionary

The difference depends on a single character:

The colon :

Understanding this distinction is fundamental if you want to truly master Python’s core data structures.

Without : → It’s a Set

If you use curly braces without a colon, Python creates a set.

numbers = [1, 2, 2, 3, 3, 3]

my_set = {n for n in numbers}
print(my_set)
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Output:

{1, 2, 3}
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Why?

A set:

  • Stores unique values only
  • Automatically removes duplicates
  • Is unordered
  • Does not use key-value pairs

In this example, repeated values (2, 3) are discarded because sets enforce uniqueness.

With : → It’s a Dictionary

If you include a colon inside {}, Python creates a dictionary.

numbers = [1, 2, 2, 3, 3, 3]

my_dict = {n: n**2 for n in numbers}
print(my_dict)
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Output:

{1: 1, 2: 4, 3: 9}
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How to Read This Line 

{n: n**2 for n in numbers}
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You can read it as:

"Create a dictionary where the key is n,

the value is n**2,

for each n in numbers."

Structure breakdown:

{ key : value }
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  • The first n → becomes the key
  • n**2 → becomes the value
  • for n in numbers → iterates over the list

Why Are There Only 3 Key-Value Pairs?

The original list contains 6 elements:

[1, 2, 2, 3, 3, 3]
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But the resulting dictionary has only 3 pairs.

Why?

Because dictionary keys must be unique.

When Python processes this:

  • First 2 → creates {2: 4}
  • Second 2 → overwrites {2: 4}
  • Same happens with 3

Dictionaries do not allow duplicate keys. If a key appears again, its value is overwritten.

That’s why the final result contains only:

{1: 1, 2: 4, 3: 9}
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Important Edge Case: Empty Braces

One subtle but important detail:

empty = {}
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This creates a dictionary, not a set.

To create an empty set, you must use:

empty_set = set()
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Quick Comparison

Syntax Creates Purpose
{x} Set Store unique values
{x: y} Dictionary Map keys to values
{} Dictionary Empty dict
set() Set Empty set

Final Rule

If there is a colon : inside {}, you're creating a dictionary.

If there is no colon, you're creating a set.

That single character completely changes the meaning of the structure.

Small syntactic details like this are what separate someone who uses Python from someone who truly understands Python.

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