Python3 how to raise a ValueError for a date time object if its not in the right format?

Question

For a uni project we have to create a program conforming to some tests that we have been given. The tests use magic numbers as inputs for the functions. I know how to get it to return a datetime object. Just dont know how to raise the error!

my code:

    import datetime
    def time_to_datetime(datetimeSTR):
        datetime_obj = datetime.datetime.strptime(datetimeSTR, '%Y/%m/%d %H:%M')
        if datetimeSTR != datetime.datetime.strptime(datetimeSTR, '%Y/%m/%d %H:%M'):
            raise ValueError('The time must have the format YYYY/MM/DD HH:MM')
        else:
            return datetime_obj

Test Code:

    import datetime
    import os
    import unittest   # Standard unittest framework.

    import utils   # The module implementing JourneyOptions class.


    class TestTimeToDatetime(unittest.TestCase):
        """Tests for the time_to_datetime function."""

        def test_invalid_time_is_rejected(self):
            with self.assertRaises(ValueError) as cm:
                utils.time_to_datetime('2019/06/09 12:60')
            self.assertEqual(
                'The time must have the format YYYY/MM/DD HH:MM',
                str(cm.exception))

        def test_valid_time_yields_a_dattime_object(self):
            d = utils.time_to_datetime('2019/06/09 12:59')
            self.assertTrue(isinstance(d, datetime.datetime))

these are the results i get:

======================================================================

ERROR: test_valid_time_yields_a_dattime_object (__main__.TestTimeToDatetime)

Traceback (most recent call last):
  File "C:/Users/s5115426/Desktop/tests/test_utils.py", line 21, in test_valid_time_yields_a_dattime_object
    d = utils.time_to_datetime('2019/06/09 12:59')
  File "C:\Users\s5115426\Desktop\tests\utils.py", line 25, in time_to_datetime
    raise ValueError('The time must have the format YYYY/MM/DD HH:MM')
ValueError: The time must have the format YYYY/MM/DD HH:MM

======================================================================
FAIL: test_invalid_time_is_rejected (__main__.TestTimeToDatetime)

Traceback (most recent call last):
  File "C:/Users/s5115426/Desktop/tests/test_utils.py", line 18, in test_invalid_time_is_rejected
    str(cm.exception))
AssertionError: 'The time must have the format YYYY/MM/DD HH:MM' != 'unconverted data remains: 0'
- The time must have the format YYYY/MM/DD HH:MM
+ unconverted data remains: 0

Any help would be much appreciated!

Answer 1

The error that you are making is that strptime raises a value error if the date string does not match the given format. for example:

date_str = '2019/06/09 12:60'
datetime_obj = datetime.datetime.strptime(date_str, '%Y/%m/%d %H:%M')
.
.
ValueError: unconverted data remains: 0

datetime_obj never gets set because strptime never returns a value.

Also, as noted, you are comparing the response from strptime to the inout string -- that will never work.

Try a simpler approach:

import datetime
def time_to_datetime(datetimeSTR):
    datetime_obj = datetime.datetime.strptime(datetimeSTR, '%Y/%m/%d %H:%M')
    return datetime_obj

Or you can catch any exception and reraise (imo, bad practice, but might be worth doing to illustrate.

import datetime
def time_to_datetime(datetimeSTR):
    try:
        datetime_obj = datetime.datetime.strptime(datetimeSTR, '%Y/%m/%d %H:%M')
    except ValueErr as err:
        print(err)
        raise ValueError('The time must have the format YYYY/MM/DD HH:MM')
    else:
        return datetime_obj

This way, you see how strptime behaves for various inputs in your test and you have error handling in your function .... though, as I indicated, reraising an error is (imo) bad practice.