مدير السياق: مع و __أدخل__/__خروج__

with open("note.txt", "w") as f:
    f.write("ciao")
# qui f è già chiuso, anche se write() ha sollevato un'eccezione

with open("a.txt") as f, open("b.txt") as g:
    dati_a = f.read()
    dati_b = g.read()

class Timer:
    def __enter__(self):
        import time
        self.t0 = time.perf_counter()
        return self  # è l'oggetto che finisce dopo "as"
    def __exit__(self, exc_type, exc_value, traceback):
        import time
        self.elapsed = time.perf_counter() - self.t0
        # ritornare True ingoia l'eccezione; di solito non lo vuoi
        return False

with Timer() as t:
    sum(range(10**6))
t.elapsed  # quanto è durato

from contextlib import contextmanager

@contextmanager
def timer():
    import time
    t0 = time.perf_counter()
    try:
        yield  # qui sta il blocco "with"
    finally:
        elapsed = time.perf_counter() - t0
        print(f"durata: {elapsed:.4f}s")

with timer():
    sum(range(10**6))

from contextlib import suppress

with suppress(FileNotFoundError):
    open("forse-non-esiste.txt").read()
# se il file non c'è, nessun errore propaga

x = 42
with x:  # AttributeError: __enter__
    ...

from contextlib import contextmanager

@contextmanager
def manage_resource():
    # __enter__ code
    yield resource
    # __exit__ code