sample solution: write a negative test

test_times.py

import pytest
from times import compute_overlap_time, time_range

# two ways of doing the same negative test. Only one is needed.
def test_negative_time_range():
    # first possible solution
    with pytest.raises(ValueError) as e:
        time_range("2010-01-12 10:00:00", "2010-01-12 09:30:00")
    # lines after the error is raised are not executed in the pytest.raises context, so the assertion has to be outside the "with"    
    assert e.match('The end of the time range has to come strictly after its start.')
    
def test_negative_time_range_alternative():
    # an alternative solution for using pytest.raises to check that the error message is as expected
    expected_error_message = 'The end of the time range has to come strictly after its start.'
    with pytest.raises(ValueError, match=expected_error_message):
        time_range("2010-01-12 10:00:00", "2010-01-12 09:30:00")

times.py

import datetime


def time_range(start_time, end_time, number_of_intervals=1, gap_between_intervals_s=0):
    start_time_s = datetime.datetime.strptime(start_time, "%Y-%m-%d %H:%M:%S")
    end_time_s = datetime.datetime.strptime(end_time, "%Y-%m-%d %H:%M:%S")
    if(start_time_s>=end_time_s):
        raise ValueError('The end of the time range has to come strictly after its start.')
    d = (end_time_s - start_time_s).total_seconds() / number_of_intervals + gap_between_intervals_s * (1 / number_of_intervals - 1)
    sec_range = [(start_time_s + datetime.timedelta(seconds=i * d + i * gap_between_intervals_s),
                  start_time_s + datetime.timedelta(seconds=(i + 1) * d + i * gap_between_intervals_s))
                 for i in range(number_of_intervals)]
    return [(ta.strftime("%Y-%m-%d %H:%M:%S"), tb.strftime("%Y-%m-%d %H:%M:%S")) for ta, tb in sec_range]


def compute_overlap_time(range1, range2):
    overlap_time = []
    for start1, end1 in range1:
        for start2, end2 in range2:
            # both ranges need to start before the other ends, otherwise there is no overlap!
            if start1 <= end2 and start2 <= end1: 
                low = max(start1, start2)
                high = min(end1, end2)
                if high == low:
                    continue
                overlap_time.append((low, high))
    return overlap_time