14_context_manager.py

# Context managers: __enter__/__exit__ are what "with" calls.


class Timer:
    def __enter__(self):
        import time
        self.start = time.monotonic()
        # whatever __enter__ returns becomes the "as" variable
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        import time
        self.elapsed = time.monotonic() - self.start
        print(f"took {self.elapsed:.4f}s")
        # returning True here would SWALLOW the exception; False re-raises
        return False


with Timer():
    sum(range(1_000_000))


# The same thing with far less code — contextlib turns a generator into
# a context manager: everything before yield is __enter__, after is __exit__.
from contextlib import contextmanager


@contextmanager
def tag(name):
    print(f"<{name}>")
    yield
    print(f"</{name}>")


with tag("h1"):
    print("Hello")