List and Dictionary Comprehensions

Definition

Can a whole loop fit in one readable line?

In simple words

Instead of making an empty list and appending in a loop, say what you want: [n * n for n in range(5)] means 'the square of n, for each n from 0 to 4'. Add if to filter: [n for n in marks if n >= 40] keeps only passing marks.

Why it matters

Comprehensions are the everyday Python way to transform and filter data: cleaning a list of words, scaling numbers, building a lookup table. You will see them in almost every data and AI script.

How it works

# The loop
squares = []
for n in range(5):
    squares.append(n * n)

# The same, as a comprehension
squares = [n * n for n in range(5)]

In code

Squares, a filter, cleaned words and a dictionary built in one line each.

squares = [n * n for n in range(1, 6)]
print("Squares:", squares)

marks = [35, 72, 48, 91, 20]
passed = [m for m in marks if m >= 40]
print("Passed:", passed)

raw = ["  Mango", "banana ", " GUAVA "]
clean = [fruit.strip().lower() for fruit in raw]
print("Clean:", clean)

lengths = {fruit: len(fruit) for fruit in clean}
print("Lengths:", lengths)

even_squares = {n: n * n for n in range(10) if n % 2 == 0}
print("Even squares:", even_squares)

Squares: [1, 4, 9, 16, 25]
Passed: [72, 48, 91]
Clean: ['mango', 'banana', 'guava']
Lengths: {'mango': 5, 'banana': 6, 'guava': 5}
Even squares: {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

Try it yourself

Common mistakes

• Putting the if in the wrong place. A filter goes at the end: [x for x in xs if x > 0].
• Writing very long comprehensions that nobody can read.
• Using a comprehension just to call a function for its side effect, such as printing. Use a loop.

Check yourself

Summary

• Comprehensions build lists, sets and dictionaries in one expression.
• Add if at the end to filter.
• Prefer a normal loop when the comprehension gets hard to read.