[Zope3-dev] Proposing to simplify datetime

[Tim Peters]

The current datetime module class hierarchy looks like:

object
    timedelta
    tzinfo
    time
        timetz
    date
        datetime
            datetimetz

Several people have noted (on the wiki, and in email), with some
bewilderment, that:

1. A timetz object with tzinfo=None acts like a time object.

2. A datetimetz object with tzinfo=None acts like a datetime object.

A natural suggestion is to change the hierarchy to:

object
    timedelta
    tzinfo
    time
    date
        datetime

where the current time and datetime classes go away, the current timetz is
renamed to time, and the current datetimetz to datetime.

I tried that this evening for the Python implementation, and it was easy
enough.  No subtle problems popped up.  The results are checked in to the
Python datetime sandbox, as datetime2.py and test_datetime2.py; it appears
to be fully functional, and allowed tossing about 10% of the code lines.

IIRC, the original motivation for making a finer distinction among objects
with and without tzinfo members was to save memory in the objects.  Guido
thinks that's a red herring, though, and I agree.  The C implementation of
datetime objects currently has unused "pad bytes" in the structs (due to
compiler alignment of struct members), and adding a flag byte to record
"does this have a non-None tzinfo member or not?" wouldn't actually consume
any more memory.

The C implementation would still need to have more than one way to allocate
a time/datetime under the covers, in order to avoid allocating an additional
pointer field (for the PyObject* tzinfo pointer) when it wasn't needed, so
it wouldn't really simplify the C implementation much.  It would simplify
the user's mental model, and cut out a ton of near-redundant docs (those two
are pretty much the same thing <wink>).