Dict and Set Comprehensions

You've used list comprehensions to build lists in one line. Python has the same syntax for dicts and sets.

Dict comprehension syntax compared to list comprehension:

# List comprehension:
[expr for item in iterable]
# Dict comprehension:
{key_expr: val_expr for item in iterable}

Python

Here, we build a dict that maps each fruit to its length:

fruits = ['apple', 'banana', 'cherry']
lengths = {f: len(f) for f in fruits}
print(lengths)

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Output

You can pair two lists using zip() inside a dict comprehension:

names = ['Alice', 'Bob', 'Charlie']
scores = [85, 92, 78]
d = {n: s for n, s in zip(names, scores)}
print(d)

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Output

words = ['hi', 'hello', 'hey']
result = {w: len(w) for w in words}
print(result['hi'])

Python

Filter to only include words longer than 3 characters:

words = ['hi', 'hello', 'hey', 'world']
long = {w: len(w) for w in words if len(w) > 3}
print(long)

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Output

s = {'a': 90, 'b': 55, 'c': 80}
p = {k: v for k, v in s.items() if v >= 70}
print(len(p))

Python

Set comprehensions look like dict comprehensions but use a single expression. The result is a set, so duplicates are removed automatically.

Set comprehension syntax:

# List comprehension (keeps duplicates):
[expr for item in iterable]
# Set comprehension (removes duplicates):
{expr for item in iterable}

Python

Get the set of unique lengths from a list of words:

words = ['cat', 'dog', 'bee', 'elephant']
lengths = {len(w) for w in words}
print(sorted(lengths))

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Output

letters = ['a', 'b', 'a', 'c', 'b', 'a']
result = {c for c in letters}
print(len(result))

Python

Set comprehensions also support filtering with an if clause:

nums = [1, 2, 3, 4, 5, 6]
evens = {n for n in nums if n % 2 == 0}
print(sorted(evens))

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Output

words = ['a', 'bb', 'ccc']
d = {w: len(w) for w in words}
print(d['bb'])

Python

data = {'x': 10, 'y': 5, 'z': 8}
big = {k: v for k, v in data.items() if v > 7}
print(sorted(big.keys()))

Python

items = [('a', 1), ('b', 2), ('c', 3)]
d = {k: v for k, v in items}
print(d['b'])

Python