What are generators and how to use them?

Question

Accepted Answer

**Generator** is a function that **returns values one at a time** using `yield`, saving memory.

**Generator function:**
```python
def count_up_to(n):
    count = 1
    while count <= n:
        yield count  # Returns value, pauses
        count += 1

# Usage
for num in count_up_to(5):
    print(num)  # 1, 2, 3, 4, 5
```

**Generator expression:**
```python
# Like list comprehension, but with ()
squares = (x**2 for x in range(10))

for s in squares:
    print(s)
```

**Advantages:**
- **Memory efficient** — values are generated on the fly.
- **Lazy evaluation** — next value only when requested.
- Can work with infinite sequences.

```python
# Infinite sequence
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

fib = fibonacci()
print(next(fib))  # 0
print(next(fib))  # 1
print(next(fib))  # 1
```

Python: What are generators and how to use them?