[Python-Dev] (#19562) Asserts in Python stdlib code (datetime.py)
alexander.belopolsky at gmail.com
Fri Dec 13 18:56:14 CET 2013
On Fri, Nov 15, 2013 at 9:10 PM, Tim Peters <tim.peters at gmail.com> wrote:
> > _DI4Y = _days_before_year(5)
> > # A 4-year cycle has an extra leap day over what we'd get from pasting
> > # together 4 single years.
> > assert _DI4Y == 4 * 365 + 1
> > To me, the constant should be directly set to its known value.
> > _DI4Y = 4*365 + 1.
> > The function should then be tested in test_datetime.
> > self.assertEqual(dt._days_before_year(5), dt._DI4Y)
> I think making that change would be pointless code churn. Harmful,
> even. As the guy who happened to have written that code ;-), I think
> it's valuable to have the _code_ (not off buried in some monstrously
> tedious test file) explain what the comments there do explain, and
> verify with the assert. If anyone needs to muck with the
> implementation of datetime, it's crucial they understand what DI4Y
> _means_, and that it's identical to _days_before_year(5). Its actual
> value (4*365 + 1) isn't really interesting. Defining _DI4Y _as_
> _days_before_year(5) captures its _meaning_.
Interestingly, the corresponding C code is closer to what Terry suggested:
/* Number of days in 4, 100, and 400 year cycles. That these have
* the correct values is asserted in the module init function.
#define DI4Y 1461 /* days_before_year(5); days in 4 years */
#define DI100Y 36524 /* days_before_year(101); days in 100 years */
#define DI400Y 146097 /* days_before_year(401); days in 400 years */
... skipping to the init function ...
/* A 4-year cycle has an extra leap day over what we'd get from
* pasting together 4 single years.
assert(DI4Y == 4 * 365 + 1);
assert(DI4Y == days_before_year(4+1));
This is probably explainable by the limitations of the C language, but I
_DI4Y = 4 * 365 + 1
assert _DI4Y == _days_before_year(5)
to be more natural than the way it is currently written.
> Ain't broke - don't fix.
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