Dict and set comprehension

Learn {k: v for ...} and {x for ...}.

Dict comprehension

A dict comprehension builds a dictionary in one line. It uses curly braces and a colon:

squares = {n: n * n for n in [1, 2, 3]}
# {1: 1, 2: 4, 3: 9}

The shape is: {key: value for variable in list}. The key and value are computed for each item. If a key appears twice, the later one overwrites the earlier.

Set comprehension

A set comprehension also uses curly braces but no colon:

unique = {x % 3 for x in [1, 2, 4, 7]}
# {0, 1, 2}

The result is a set: each value appears once. The only difference from a list comprehension is the curly brackets.

A set is unordered and unique; if order matters, use a list.

Exercise Write a squares dict

Assign to squares a dict comprehension that holds n: n * n pairs for each n in [1, 2, 3, 4]. Then print print(squares).

Show example solution

squares = {n: n * n for n in [1, 2, 3, 4]}
print(squares)

Test yourself

Which is a dict comprehension?
- [k: v for k, v in pairs]
- (k: v for k, v in pairs)
- {k: v for k, v in pairs}
- <k, v for k, v in pairs>
Show answer

{k: v for k, v in pairs} — Curly braces and a colon mark a dict comprehension.
Match the comprehension types

[...]

List
{k: v}

Dict
{x}

Set
(...)

Generator

Complete the set comprehension

unique = {x % 3 for x in [____, 2, 4, 7]}

Show answer

unique = {x % 3 for x in [1, 2, 4, 7]}

Next lesson: Nested comprehension

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