Comprehension basics
- Python course
- Beginner
- Lesson 59/116
- 5 min
Learn comprehensions with [x for x in xs].
Comprehension basics
List comprehension is a short and readable way to build a list. Instead of a loop and append, you write it in one line:
squares = [n * n for n in [1, 2, 3]]
# [1, 4, 9]The shape is: [expression for variable in list]. The expression is computed for each item and added to the result. This is shorter and faster than a classic for loop.
Prefer comprehension when it stays readable; if it grows too complex, a loop may be clearer.
Comprehension replaces a for loop
You can get the same result with a classic for loop:
squares = []
for n in [1, 2, 3]:
squares.append(n * n)Comprehension reduces these three lines to one. Which one to use is a matter of taste and readability.
In nested loops, comprehension can become complex; then a for loop may be clearer.
Practice Write a tripling comprehension
Assign to triples a comprehension that multiplies each item of [1, 2, 3, 4] by 3. Then print print(triples).
Starter code
# Triple with a comprehensionSample solution
triples = [n * 3 for n in [1, 2, 3, 4]]
print(triples)Expected output
[3, 6, 9, 12]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which produces the list [2, 4, 6]?
- [x * 2 for x in [1, 2, 3]] Correct answer
- [x for x in [2, 4, 6]]
- [x + 2 for x in [1, 2, 3]]
- [x * 2 for x in [2, 4, 6]]
Explanation: [x * 2 for x in [1, 2, 3]] doubles each item: [2, 4, 6].
2 Complete the comprehension
doubles = [n * ____ for n in [1, 2, 3]]doubles = [n * 2 for n in [1, 2, 3]]Explanation: The expression needs a constant multiplier.
3 Match the comprehension parts
| Term | Meaning |
|---|---|
[...] |
List comprehension |
for |
Loop keyword |
expression |
Per-item computation |
in |
Source keyword |