Ekspresi pembangkit

A generator expression is like a list comprehension, but with parentheses instead of square brackets. The difference is dramatic: it doesn't build a list in memory, it produces elements on-demand, one at a time.

gen = (n * n for n in range(1_000_000))
# istantaneo, memoria costante: nessun calcolo ancora fatto

list_comp = [n * n for n in range(1_000_000)]
# alloca 1M di int in memoria

Laziness (lazy evaluation)

The generator does nothing until someone asks for the next value:

gen = (n * n for n in range(5))
next(gen)    # 0    (calcolato adesso)
next(gen)    # 1
next(gen)    # 4
# ... e così via, fino a StopIteration

It gets exhausted: after consuming all elements, it is empty forever. To reuse it, you must recreate it.

When to use it

When you pass the results to a function that consumes them in sequence:

total = sum(n * n for n in range(1_000_000))
# nessuna lista intermedia, memoria costante

sum, any, all, max, min, "".join(...) all accept generators. If the outer function only requires the call parentheses, you can omit the generator's own:

sum((n * n for n in range(10)))    # OK
sum(n * n for n in range(10))      # IDENTICO, più leggibile

any and all: short-circuit

any(...) and all(...) stop iterating as soon as they have the answer. With a generator, the following elements aren't even computed.

nums = [1, 2, 3, 4, 5]
any(n > 3 for n in nums)    # True (si ferma a 4)
all(n > 0 for n in nums)    # True
all(n > 2 for n in nums)    # False (si ferma a 1)

Pattern: lazy pipeline

righe = (linea.strip() for linea in testo.splitlines())
non_vuote = (r for r in righe if r)
prime_dieci = []
for r in non_vuote:
    prime_dieci.append(r)
    if len(prime_dieci) == 10:
        break

No intermediate list, even on huge inputs.

Consuming generators with sum, any, all

Generator expressions are perfect for passing directly to functions that consume iterables, like sum(), any(), or all(). In this case, you can omit the outer parentheses of the generator expression, simply writing:

squares_sum = sum(x**2 for x in range(10))

This keeps your code clean and highly memory-efficient.

Try it

Review exercise

Additional challenge