How to mock/set system date in pytest?

Question

In some of my tests I am having a problem that they fail on Travis because of time and time zone problems, so I want to mock system time for my test. How can I do this?

Answer 1

There are two ways you can accomplish that:

1. Create function which you will call instead of datetime.datetime.now() as suggested by Bruno, but here is different implementation:

   import os
   import datetime
   
   def mytoday():
    if 'MYDATE' in os.environ:
        return datetime.datetime.strptime(os.getenv('MYDATE'), '%m-%d-%Y').date()
    else:
        return datetime.date.today()
   

   Then, in your test, you just monkeypatch environment variable:

   import datetime
   
   def test_patched_date(monkeypatch):
       monkeytest.setenv('MYDATE', '05-31-2014')
       assert datetime.date.today() == datetime.date(2014, 5, 31)
   

2. Monkeypatch the datetime function:

   import datetime
   import pytest
   
   FAKE_TIME = datetime.datetime(2020, 12, 25, 17, 05, 55)
   
   @pytest.fixture
   def patch_datetime_now(monkeypatch):
   
       class mydatetime:
           @classmethod
           def now(cls):
               return FAKE_TIME
   
       monkeypatch.setattr(datetime, 'datetime', mydatetime)
   
   
   def test_patch_datetime(patch_datetime_now):
       assert datetime.datetime.now() == FAKE_TIME