What is a generator expression vs list comprehension?

Question

Accepted Answer

**Generator expression** produces values lazily. **List comprehension** creates the entire list in memory.

```python
# List comprehension — creates full list
squares_list = [x**2 for x in range(10_000_000)]
# Uses ~400MB of memory

# Generator expression — produces one at a time
squares_gen = (x**2 for x in range(10_000_000))
# Uses ~200 bytes

# Consuming
for s in squares_gen:
    process(s)  # Each value generated on-the-fly

# Converting to list when needed
result = list(squares_gen)
```

**Key differences:**
- **Memory**: generator is O(1), list is O(n).
- **Speed**: list is faster for single iteration.
- **Iteration**: generator can only iterate once.
- **Indexing**: list supports `[]`, generator does not.

**When to use generators:**
- Large datasets.
- Streaming data.
- Pipelines with `itertools`.

Python: What is a generator expression vs list comprehension?