A simple timer implementation that supports clock injection

timer.py

import time
from datetime import datetime

class Clock:
    def now(self):
        return datetime.now()


class Timer:
                                       # or a simpler alternative...
    def __init__(self, clock=None):    # def __init__(self, now=datetime.now):
        if clock is None:              #   self._now = now
            clock = Clock()

        self.clock = clock
        self._start = None
        self._stop = None
        self._elapsed_time = None

    def start(self):
        self._start = self.clock.now() # self._start = self._now()

    def stop(self):
        self._stop = self.clock.now()  # self._stop = self._now()

    @property
    def elapsed_time(self):
        if not self._start:
            raise ValueError("Timer not started")

        if self._stop:
            now = self._stop
        else:
            now = self.clock.now()     # now = self._now()

        self._elapsed_time = now - self._start
        return self._elapsed_time


t = Timer()
t.start()
time.sleep(2)
t.stop()
print(t.elapsed_time)  # prints ~0:00:02

class FakeClock:
    def now(self):
        return datetime(year=2022, month=12, day=28, hour=0, minute=0)

t = Timer(FakeClock())
t.start()
time.sleep(2)
t.stop()

print(t.elapsed_time)  # prints 0:00:00