How can I unit test a method using a datetime?

Question

I have the follow class and method:

class DateTimeHelper(object):

    @staticmethod
    def get_utc_millisecond_timestamp():
        (dt, micro) = datetime.datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S.%f').split('.')
        return "%s.%03d" % (dt, int(micro) / 1000)  # UTC time with millisecond

How can I unit test it? I am completely stumped although this is simple. It's my first unit test.

Answer 1

Use the unittest.mock library (Python 3.3 and newer, backported as mock), to replace calls to any code external to your code-under-test.

Here, I'd mock out not only utcnow() but strftime() too, to just return a string object:

with mock.patch('datetime.datetime') as dt_mock:
    dt_mock.utcnow.return_value.strftime.return_value = '2016-08-04 12:22:44.123456'
    result = DateTimeHelper.get_utc_millisecond_timestamp()

If you feel that testing the strftime() argument is important, give dt_mock.utcnow.return_value an explicit datetime object to return instead; you'd have to create that test object before you mock however, as you can't mock out just the datetime.datetime.utcnow class method:

testdt = datetime.datetime(2016, 8, 4, 12, 22, 44, 123456)
with mock.patch('datetime.datetime') as dt_mock:
    dt_mock.utcnow.return_value = testdt
    result = DateTimeHelper.get_utc_millisecond_timestamp()

or, in your unittests, use from datetime import datetime to keep a reference to the class that isn't mocked.

Demo:

>>> from unittest import mock
>>> import datetime
>>> class DateTimeHelper(object):
...     @staticmethod
...     def get_utc_millisecond_timestamp():
...         (dt, micro) = datetime.datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S.%f').split('.')
...         return "%s.%03d" % (dt, int(micro) / 1000)  # UTC time with millisecond
...
>>> with mock.patch('datetime.datetime') as dt_mock:
...     dt_mock.utcnow.return_value.strftime.return_value = '2016-08-04 12:22:44.123456'
...     result = DateTimeHelper.get_utc_millisecond_timestamp()
...
>>> result
'2016-08-04 12:22:44.123'
>>> testdt = datetime.datetime(2016, 8, 4, 12, 22, 44, 123456)
>>> with mock.patch('datetime.datetime') as dt_mock:
...     dt_mock.utcnow.return_value = testdt
...     result = DateTimeHelper.get_utc_millisecond_timestamp()
...
>>> result
'2016-08-04 12:22:44.123'