How to mock python's datetime.now() in a class method for unit testing?

Question

I'm trying to write tests for a class that has methods like:

import datetime
import pytz

class MyClass:
    def get_now(self, timezone):
        return datetime.datetime.now(timezone)

    def do_many_things(self, tz_string='Europe/London'):
        tz = pytz.timezone(tz_string)
        localtime_now = self.get_now(tz)
        ...
        return things

I want to test it, and to do so I need to make sure that the datetime.datetime.now() call returns something predictable.

I've been reading lots of examples of using Mock in tests, but haven't found anything quite like what I need, and I can't work out how to use it in my tests.

I separated the get_now() method out in case it's easier to mock that, instead of datetime.datetime.now(), but I'm still stumped. Any thoughts on how to write UnitTests for this using Mock? (This is all in Django, fwiw; I'm not sure if this makes a difference in this case.)

Solution 1

You could use freezegun :

from freezegun import freeze_time

def test():
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)
    with freeze_time("2012-01-14"):
        assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)

It basically mocks datetime module calls.

Solution 2

You'd create a function that returns a specific datetime, localized to the timezone passed in:

import mock

def mocked_get_now(timezone):
    dt = datetime.datetime(2012, 1, 1, 10, 10, 10)
    return timezone.localize(dt)

@mock.patch('path.to.your.models.MyClass.get_now', side_effect=mocked_get_now)
def your_test(self, mock_obj):
    # Within this test, `MyClass.get_now()` is a mock that'll return a predictable
    # timezone-aware datetime object, set to 2012-01-01 10:10:10.

That way you can test if the resulting timezone-aware datetime is correctly being handled; results elsewhere should show the correct timezone but will have a predictable date and time.

You use the mocked_get_now function as a side-effect when mocking get_now; whenever code calls get_now the call is recorded by mock, and mocked_get_now is called, and it's return value used as the value returned to the caller of get_now.

Solution 3

I'm using date, but the same idea should work for datetime:

class SpoofDate(date):
    def __new__(cls, *args, **kwargs):
        return date.__new__(date, *args, **kwargs)

...

from mock import patch

@patch('some.module.date', SpoofDate)
def testSomething(self):
    SpoofDate.today = classmethod(lambda cls : date(2012, 9, 24))

Where some.module imports date. Patch is replacing the imported date with SpoofDate, which you can then redefine to do whatever you want.

Solution 4

Here's the most elegant way to do this IMO:

import datetime
from unittest import mock

test_now = datetime.datetime(1856, 7, 10)
with mock.patch('datetime.datetime', wraps=datetime.datetime) as dt:
    print(dt.now()) # calls the real thing
    dt.now.return_value = test_now
    print(dt.now()) # calls the mocked value

The advantages here is that you needn't patch the datetime module via the tested module's local attribute, it supports calling unmocked methods, and it doesn't require any external imports.

Solution 5

Having asked this question originally...

As @Jocelyn delalande suggested, I've been happily using freezegun for years now.

Another option is python-libfaketime, which can be much faster than freezegun, but doesn't work on Windows and sounds a bit fiddly.

A newer option is time-machine, introduced in this blog post that compares the three options.

View more solutions

Comments

[zakiakhmad]

@FearlessFuture did you just replace date with datetime? Can you write the implementation in datetime.

[FearlessFuture]

@zakiakhmad, below is an example of what I did: class StubDate(datetime.datetime): pass @mock.patch("friend.datetime.datetime", StubDate) def test_generate_date(self): # Make datetime.datetime.now return a fixed value StubDate.now = classmethod(lambda cls: datetime.datetime(2015, 03, 11, 11, 01)) self.assertEqual( self.friend_obj.generate_date(input), datetime.datetime(2015, 03, 11, 11, 01)) > Blockquote

[zakiakhmad]

@FearlessFuture thanks a lot! I wrote the gist here for a better readability. gist.github.com/za/2a217c47582737f88259

[Avinash Raj]

@MartijnPieters can't able to import mock

[Martijn Pieters]

@AvinashRaj mock is an add-on package, install it with pip. With Python 3 it is included as unittest.mock.

[Montaro]

Thanks, It works! However the first assertion did not because the month token was not accepted in this format: 01 but datetime.datetime(2012, 1, 14) works.

[Andrey Belyak]

freezegun is slow, especially if you test logic with multiple calls to datetime.now()

[Andrey Belyak]

Thank you, works much better and simpler than any other solutions