Coders' Halloween: Tests That Fail One Day Every Four Years

Some code looks harmless but has hidden bugs lurking in its nether regions. Code that handles dates is notorious for this, and this being February 29th (the coders' halloween) it's time for the bugs to come crawling out of the woodwork.

Some of our tests were constructing expected dates, one year from today, with the following code:

from datetime import datetime

now = datetime.utcnow()
then = datetime(now.year + 1, now.month, now.day)

Of course if you run this code today, then it tries to construct a datetime for February 29th 2013, which fails because that date doesn't exist.

When I posted this on twitter a few people suggested that instead we should have used timedelta(days=365) instead. Again, this works most of the time - but if you want a date exactly one year from now it will fail in leap years when used before February 29th:

>>> from datetime import datetime, timedelta
>>> datetime(2012, 2, 27) + timedelta(days=365)
datetime.datetime(2013, 2, 26, 0, 0)

The correct fix is to use the wonderful dateutil module, in particular the dateutil.relativedelta.relativedelta:

>>> from datetime import datetime
>>> from dateutil.relativedelta import relativedelta
>>> datetime.utcnow() + relativedelta(years=1)
datetime.datetime(2013, 2, 28, 15, 20, 21, 546755)
>>> datetime(2012, 2, 27) + relativedelta(years=1)
datetime.datetime(2013, 2, 27, 0, 0)

And as another hint, always use datetime.utcnow() instead of datetime.now() to avoid horrible timezone nightmares (exactly which timezone are your servers in?).


Source: http://www.voidspace.org.uk/python/weblog/arch_d7_2012_02_25.shtml