List comprehension vs for loop in Python: which is better?

Question

Is `[x * 2 for x in nums if x > 0]` better than a normal for loop with `append`? My code reviewer keeps suggesting comprehensions.

Accepted Answer

For **building a new list from an existing iterable**, comprehensions are idiomatic, more concise, and a bit faster (no repeated `.append` attribute lookup).

```python
positives_doubled = [x * 2 for x in nums if x > 0]
```

Use a regular loop when:
- There are **side effects** (printing, writing to a DB). Don't use a comprehension just to loop.
- The logic needs several statements, `try/except`, or would need nested conditions that hurt readability.

For large data you only iterate once, use a **generator expression** instead. It doesn't build the whole list in memory:

```python
total = sum(x * 2 for x in nums if x > 0)
```

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