[Datetime-SIG] PEP-0500 (Alternative datetime arithmetic) Was: PEP 495 ... is ready ...

Tim Peters tim.peters at gmail.com
Wed Aug 19 07:52:39 CEST 2015

> ...
> This  discussion sounds overly abstract. ISTM that d(x, y) in timeline
> arithmetic can be computed as x.timestamp() - y.timestamp(), (and converting
> to a timedelta).

As someone else might say, if you want timestamps, use timestamps ;-)

I want to discourage people from thinking of it that way, because it
only works in a theoretical framework abstracting away how arithmetic
actually behaves.  Timestamps in Python suck in a world of
floating-point pain that I tried hard to keep entirely out of datetime
module semantics (although I see float operations have increasingly
wormed their way in).

Everyone who thinks about it soon realizes that a datetime simply has
"too many bits" to represent faithfully as a Python float, and so also
as a Python timestamp.  But I think few realize this isn't a problem
confined to datetimes only our descendants will experience.  It can
surprise people even today.  For example, here on my second try:

>>> d = datetime.now()
>>> d
datetime.datetime(2015, 8, 18, 23, 8, 54, 615774)
>>> datetime.fromtimestamp(d.timestamp())
datetime.datetime(2015, 8, 18, 23, 8, 54, 615773)

See?  We can't even expect to round-trip faithfully with current
datetimes.  It's not really that there "aren't enough bits" to
represent a current datetime value in a C double, it's that the
closest binary float approximating the decimal 1439957334.615774 is
strictly less than that decimal value.  That causes the microsecond
portion to get chopped to 615773 on the way back.  It _could_ be
rounded instead, which would make roundtripping work for some number
of years to come (before it routinely failed again), but rounding
would cause other surprises.

Anyway, "the right way" to think about timeline arithmetic is the way
the sample code in PEP 500 spells it:: using classic datetime
arithmetic on datetimes in (our POSIX approximation of) UTC,
converting to/from other timezones in the obvious ways  There are no
surprises then (not after PEP 495-compliant tzinfo objects exist),
neither in theory nor in how code actually behaves (leaving aside that
the results won't always match real-life clocks).

If you want to _think_ of that as being equivalent to arithmetic using
theoretical infinitely-precise Python timestamps, that's fine.  But it
also means you're over 50 years old and the kids will have a hard time
understanding you ;-)

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