#!/usr/bin/env python3

import time
from typing import Callable


class Timer(object):
    """
    A stopwatch to time how long something takes (walltime).

    e.g.

        with timer.Timer() as t:
            do_the_thing()

        walltime = t()
        print(f'That took {walltime} seconds.')
    """

    def __init__(self) -> None:
        self.start = None
        self.end = None

    def __enter__(self) -> Callable[[], float]:
        """Returns a functor that, when called, returns the walltime of the
        operation in seconds.
        """
        self.start = time.perf_counter()
        self.end = 0.0
        return lambda: self.end - self.start

    def __exit__(self, *args) -> bool:
        self.end = time.perf_counter()
        return None  # don't suppress exceptions