# Comprehensions & Nested Data — Python

Source: https://www.geekswithgeeks.com/en/python/py-comprehensions

> Build lists, dicts and sets in one line, and combine collections.

## List comprehension

List comprehensions build a new list from an iterable, optionally filtered by `if`.

```python
squares = [n * n for n in range(6)]
evens = [n for n in range(20) if n % 2 == 0]
print(squares)
print(evens)
```

## Dict & set comprehension

The same idea works for dicts and sets — just change the brackets.

```python
squares = {n: n * n for n in range(5)}
unique_lens = {len(w) for w in ["hi", "bye", "ok"]}
print(squares)
print(unique_lens)
```

## Nested data structures

Lists and dicts can nest: a list of dicts (records), or a dict of lists (grouped data) — very common in real programs.

**Quiz:** What does `[n for n in range(5) if n % 2 == 0]` produce?

- [ ] [1, 3]
- [x] [0, 2, 4]
- [ ] [0, 1, 2, 3, 4]

*Answer:* [0, 2, 4]. Only even numbers from 0-4 pass the `if` filter.