Generators & yield

# Generator function
def count_up(n):
    i = 0
    while i < n:
        yield i     # pause and return value
        i += 1      # resume here next time

for num in count_up(5):
    print(num, end=" ")  # 0 1 2 3 4
print()

# Generator expression (like list comprehension)
squares = (x**2 for x in range(10))  # note: () not []
print(type(squares))  # <class 'generator'>
print(list(squares))  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# Memory efficiency
import sys
list_comp = [x**2 for x in range(1000)]
gen_exp = (x**2 for x in range(1000))
print(f"List: {sys.getsizeof(list_comp)} bytes")
print(f"Generator: {sys.getsizeof(gen_exp)} bytes")

# Fibonacci generator
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

fib = fibonacci()
for _ in range(10):
    print(next(fib), end=" ")  # 0 1 1 2 3 5 8 13 21 34
print()

# yield from — delegate to sub-generator
def flatten(nested):
    for item in nested:
        if isinstance(item, list):
            yield from flatten(item)
        else:
            yield item

nested = [1, [2, 3], [4, [5, 6]], 7]
print(list(flatten(nested)))  # [1, 2, 3, 4, 5, 6, 7]