What are list/dict/set comprehensions?

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Comprehensions build a collection from an iterable in a single, declarative expression `[expr for item in iterable if condition]`:

- **List:** `[x*x for x in nums if x > 0]`
- **Set:** `{x % 3 for x in nums}` (deduplicated)
- **Dict:** `{k: v for k, v in pairs}`
- **Generator expression:** `(x*x for x in nums)` — same syntax with `()`, but **lazy** (no list built).

They read as 'what I want' rather than 'how to build it', and are usually **faster than an equivalent `for`+`append`** loop because the iteration runs in C and there's no repeated method lookup. You can nest loops (`[... for row in grid for x in row]`, read left-to-right) and add conditional filters or expressions.

Keep them readable: if you need multiple conditions, nesting three levels, or side effects, a plain loop is clearer. Comprehensions are for *building a value*, not for running side effects.

**Interview tip:** know which bracket gives which type, that `{}`-with-pairs is a dict but `{}`-with-values is a set (and

What are list/dict/set comprehensions?

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