How best to test functions which use date.today

[Lie Ryan]

Yuan HOng wrote:
> HI,
> 
> In my project I have several date related methods which I want tested for
> correctness. The functions use date.today() in several places. Since this
> could change every time I run the test, I hope to find someway to fake a
> date.today.
> 
> For illustration lets say I have a function:
> 
> 
> from datetime import date
> def today_is_2009():
>     return date.today().year == 2009
> 
> To test this I would like to write test function like:
> 
> def test_today_is_2009():
>     set_today(date(2008, 12, 31))
>     assert today_is_2009() == False
>     set_today(date(2009,1,1))
>     assert today_is_2009() == True
> 
> The first approach of achieving this purpose is to monkey patch the
> date.today like:
> 
> date.today = mytoday
> 
> But this fails with:
> 
> TypeError: can't set attributes of built-in/extension type 'datetime.date'

This is because today is an attribute. In python, we can override 
attribute access to become a function call. I don't have python right 
now, but try this:

del date.today
date.today = mytoday

> A second possibility would be to change the system date (I am running
> Linux). However the standard Python module doesn't provide a method for this
> purpose. I could use os.system to issue a date command. But I am not very
> comfortable with this since changing the system time could break something
> undesirably. Also I will then have to have root privilege to run my test.
> Besides, I will have to stop the ntp daemon so it will not inadvertently
> correct the system clock during the test period.
> 
> Is there any suggestion from the community on how best to test such
> functions?

It is a very bad idea to change the system date.