How to unit test time based functions without adding parameters

Question

I created a function that returns seconds left until the next occurrence of that time, but I came across a problem writing a unit test for it. How do people test this type of function that has a call to datetime.now() in it?

Adding another parameter (current_time) seems wrong just to test it as it changes the initial requirements of the function.

function to test is.

from datetime import datetime, time, timedelta

def get_time_left(target_time):
    '''return float of number of seconds left until the target_time'''

    if not isinstance( target_time, time ):
        raise TypeError("target_time must be datetime.time")

    curr_time = datetime.now()
    target_datetime = datetime.combine( datetime.today(), target_time )
    if curr_time > target_datetime:
        target_datetime = curr_time + timedelta(1)

    seconds_left = (curr_time - target_datetime).total_seconds()

    return seconds_left

The test for it is.

class TestDTime(unittest.TestCase):

    def test_time_left(self):
        dt_now = datetime.now()
        tm_5sec_future = ( dt_now + timedelta(0,5) ).time()
        self.assertEqual( dtime.get_time_left(tm_5sec_future), 5.0)

The result is.

======================================================================
FAIL: test_time_left (__main__.TestDTime)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/ctatest.py", line 37, in test_time_left
    self.assertEqual( dtime.get_time_left(tm_5sec_future), 5.0)
AssertionError: -4.999985 != 5.0

What is best approach to unit testing something like this without adding any args to the function?

Answer 1

You need to use mocking framework to isolate your UT from dependencies so your UT will have a consistence behavior.

Freezegun - is a good mocking library for times which I have been used.

Just install this library: pip install freezegun

The in your UT used the decorator @freeze_time as the following:

@freeze_time("2018-06-03") 
def test_time_left(self):
    dt_now = datetime.now()
    tm_5sec_future = (dt_now + timedelta(0, 5)).time()
    self.assertEqual(dtime.get_time_left(tm_5sec_future), -5.0)