section on context creation/management __enter__ __exit__

[mhyatt000]

Key difference:

  with obj:
      ...

requires obj.__enter__ / obj.__exit__.

  with obj():
      ...

works if obj.__call__() returns a context manager, such as one made by @contextmanager.

Rule of thumb: use __enter__/__exit__ for context-manager classes; use @contextmanager for short setup/teardown logic, especially factory-style helpers.

class Timer:
    """
    Timer utility. Usage:

        timer = Timer()
        with timer("foo"):
            do_something()

        timer.tick("bar")
        do_something_else()
        timer.tock("bar")

        timer.get_average_times() -> {"foo": 0.1, "bar": 0.2}
    """

    def __init__(self):
        self.reset()

    @contextmanager
    def __call__(self, key):
        self.tick(key)
        try:
            yield None
        finally:
            self.tock(key)

    def reset(self):
        self.counts = defaultdict(int)
        self.times = defaultdict(float)
        self.start_times = {}

    def tick(self, key):
        if key in self.start_times:
            raise ValueError(f"Timer is already ticking for key: {key}")
        self.start_times[key] = time.time()

    def tock(self, key):
        if key not in self.start_times:
            raise ValueError(f"Timer is not ticking for key: {key}")
        self.counts[key] += 1
        self.times[key] += time.time() - self.start_times[key]
        del self.start_times[key]

    def get_average_times(self, reset=True):
        ret = {key: self.times[key] / self.counts[key] for key in self.counts}
        if reset:
            self.reset()
        return ret