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    Chapter 38, Intermediate

    Context managers

    The `with` statement ensures that something is undone at the end, no matter what happens. Files, connections, locks, and temporary directories use the same mechanism.

    Code for this chapter: intermediario/cap38_context_managers.py

    The protocol

    Any object with __enter__ and __exit__ works with with. The __enter__ prepares and returns the resource. The __exit__ cleans up, and receives the exception data, if any:

    intermediario/cap38_context_managers.pylinhas 10 a 25
    import time


class Cronometro:
    def __enter__(self):
        self.inicio = time.perf_counter()
        return self

    def __exit__(self, tipo_exc, valor_exc, traceback):
        self.duracao = time.perf_counter() - self.inicio
        return False


with Cronometro() as cron:
    sum(range(100_000))
print(cron.duracao >= 0)
    Output
    True

    The value that the __exit__ returns decides the fate of the exception: False lets it continue, True the suppresses. The __exit__ runs even when the block fails:

    intermediario/cap38_context_managers.pylinhas 27 a 38
    class Ignorar:
    def __enter__(self):
        return self

    def __exit__(self, tipo_exc, valor_exc, traceback):
        print("saindo, erro:", tipo_exc.__name__ if tipo_exc else None)
        return tipo_exc is ZeroDivisionError


with Ignorar():
    1 / 0
print("continuou")
    Output
    saindo, erro: ZeroDivisionError
continuou

    The short form: contextmanager

    Writing a class just for that is overkill. The decorator contextlib.contextmanager transforms a generator with a single yield into a context manager. Everything before the yield is the __enter__. Everything after is the __exit__. Put the cleanup in a finally:

    intermediario/cap38_context_managers.pylinhas 43 a 56
    from contextlib import contextmanager


@contextmanager
def secao(titulo):
    print(f"[início] {titulo}")
    try:
        yield
    finally:
        print(f"[fim] {titulo}")


with secao("importação"):
    print("trabalhando")
    Output
    [início] importação
trabalhando
[fim] importação

    Ready-made tools

    The standard library already includes the most common cases. The suppress explicitly ignores a specific exception, and the TemporaryDirectory creates a folder that disappears at the end:

    intermediario/cap38_context_managers.pylinhas 61 a 73
    from contextlib import suppress
from pathlib import Path
import tempfile

with suppress(FileNotFoundError):
    Path("nao_existe.txt").unlink()
print("sem erro")

with tempfile.TemporaryDirectory() as pasta:
    arquivo = Path(pasta) / "tmp.txt"
    arquivo.write_text("oi", encoding="utf-8")
    print(arquivo.exists())
print(Path(pasta).exists())
    Output
    sem erro
True
False

    Exercise 1

    Change directory and return

    Write mudar_pasta(destino), a context manager that changes the working directory and always restores it at the end, even if the block fails.