datetime needs an "epoch" method

[tebeka]

BPO	2736
Nosy	@malemburg, @tim-one, @jribbens, @amauryfa, @mdickinson, @abalkin, @pitrou, @catlee, @vstinner, @bitdancer
Files	add-datetime-totimestamp-method.diff: Implementation of datetime.datetime.timetuple and tests. add-datetime-totimestamp-method-docs.diff datetime_totimestamp-3.patch issue2736-doc.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/abalkin'
closed_at = <Date 2011-04-25.17:03:30.218>
created_at = <Date 2008-05-01.21:03:25.299>
labels = ['type-feature', 'docs']
title = 'datetime needs an "epoch" method'
updated_at = <Date 2012-06-08.16:41:09.959>
user = 'https://github.com/tebeka'

bugs.python.org fields:

activity = <Date 2012-06-08.16:41:09.959>
actor = 'belopolsky'
assignee = 'belopolsky'
closed = True
closed_date = <Date 2011-04-25.17:03:30.218>
closer = 'belopolsky'
components = ['Documentation']
creation = <Date 2008-05-01.21:03:25.299>
creator = 'tebeka'
dependencies = []
files = ['10251', '10256', '12329', '21565']
hgrepos = []
issue_num = 2736
keywords = ['patch']
message_count = 67.0
messages = ['66045', '66140', '66532', '66539', '66601', '66610', '75723', '75899', '75900', '75902', '75903', '75904', '75912', '76003', '76324', '76327', '76329', '76331', '76332', '76340', '76344', '76345', '76351', '76352', '77650', '77651', '99545', '103875', '106229', '106230', '106249', '106251', '106252', '106254', '106255', '124197', '124203', '124204', '124225', '124230', '124231', '124237', '124245', '124248', '124252', '124255', '124256', '124257', '124259', '132695', '132697', '132818', '132977', '132994', '133008', '133009', '133011', '133037', '133039', '133053', '133056', '133058', '133072', '133207', '133245', '134395', '162533']
nosy_count = 23.0
nosy_names = ['lemburg', 'tim.peters', 'ping', 'jribbens', 'guettli', 'amaury.forgeotdarc', 'mark.dickinson', 'davidfraser', 'belopolsky', 'pitrou', 'andersjm', 'catlee', 'vstinner', 'tomster', 'werneck', 'hodgestar', 'Neil Muller', 'erik.stephens', 'steve.roberts', 'r.david.murray', 'vivanov', 'python-dev', 'Jay.Taylor']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue2736'
versions = ['Python 3.3']

[tebeka]

If you try to convert datetime objects to seconds since epoch and back
it will not work since the microseconds get lost:

>>> dt = datetime(2008, 5, 1, 13, 35, 41, 567777)
>>> seconds = mktime(dt.timetuple())
>>> datetime.fromtimestamp(seconds) == dt
False

Current fix is to do
>>> seconds += (dt.microsecond / 1000000.0)
>>> datetime.fromtimestamp(seconds) == dt
True

[werneck]

That's expected as mktime is just a thin wrapper over libc mktime() and
it does not expect microseconds. Changing time.mktime doesn't seems an
option, so the best alternative is to implement a method in datetime
type. Is there a real demand for C code implementing this to justify it?

[hodgestar]

Attached a patch which adds a .totimetuple(...) method to
datetime.datetime and tests for it.

The intention is that the dt.totimetuple(...) method is equivalent to:
mktime(dt.timetuple()) + (dt.microsecond / 1000000.0)

[hodgestar]

Patch adding documentation for datetime.totimestamp(...).

[tebeka]

I think the name is not good, should be "toepoch" or something like that.

[NeilMuller]

datetime has fromtimestamp already, so using totimestamp keeps naming
consistency (see toordinal and fromordinal).

[vstinner]

See also bpo-1673409

[vstinner]

I like the method, but I have some comments about the new method:

• datetime_totimestamp() is not well indented
• "PyObject *time" should be defined at the before the first
  instruction
• why not using "if (time == NULL) return NULL;" directly instead of
  using a block in case of time is not NULL?
• there are reference leaks: timetuple, timestamp and
  PyFloat_FromDouble()

I wrote a similar patch before reading
add-datetime-totimestamp-method.diff which does exactly the same... I
attach my patch but both should be merged.

[vstinner]

Here is a merged patch of the three patches. Except the C
implementation of datetime_totimestamp() (written by me), all code is
written by hodgestar.

[abalkin]

I would like to voice my opposition the totimestamp method.

Representing time as a float is a really bad idea (originated at
Microsoft as I have heard). In addition to the usual numeric problems
when dealing with the floating point, the resolution of the floating
point timestamp varies from year to year making it impossible to
represent high resolution historical data.

In my opinion both time.time() returning float and
datetime.fromtimestamp() taking a float are both design mistakes and
adding totimestamp that produces a float will further promote a bad
practice.

I would not mind integer based to/from timestamp methods taking and
producing seconds or even (second, microsecond) tuples, but I don't
think changing fromtimestamp behavior is an option.

[vstinner]

Le Saturday 15 November 2008 02:15:30 Alexander Belopolsky, vous avez écrit :

I don't think changing fromtimestamp behavior is an option.

It's too late to break the API (Python3 is in RC stage ;-)), but we can create
new methods like:
datetime.fromepoch(seconds, microseconds=0) # (int/long, int)
datetime.toepoch() -> (seconds, microseconds) # (int/long, int)

[abalkin]

On Fri, Nov 14, 2008 at 8:37 PM, STINNER Victor <report@bugs.python.org> wrote:

.. but we can create new methods like:
datetime.fromepoch(seconds, microseconds=0) # (int/long, int)

While 1970 is the most popular epoch, I've seen 1900, 2000 and even
2035 (!) being used as well. Similarly, nanoseconds are used in high
resolution time sources at least as often as microseconds. This makes
fromepoch() ambiguous and it is really unnecessary because it can be
written as epoch + timedelta(0, seconds, microseconds).

datetime.toepoch() -> (seconds, microseconds) # (int/long, int)

I would much rather have divmod implemented as you suggested in
bpo-2706 . Then toepoch is simply

def toepoch(d):
    x, y = divmod(d, timedellta(0, 1))
    return x, y.microseconds

[vstinner]

Le Saturday 15 November 2008 04:17:50 Alexander Belopolsky, vous avez écrit :

it is really unnecessary because it can be
written as epoch + timedelta(0, seconds, microseconds).

I tried yesterday and it doesn't work!

datetime.datetime(1970, 1, 1, 1, 0)
>>> t1 = epoch + timedelta(seconds=-1660000000)
>>> t2 = datetime.fromtimestamp(-1660000000)
>>> t2
datetime.datetime(1917, 5, 26, 1, 53, 20)
>>> t1 - t2
datetime.timedelta(0)
>>> t2 = datetime.fromtimestamp(-1670000000)
>>> t2
datetime.datetime(1917, 1, 30, 7, 6, 40)
>>> t1 = epoch + timedelta(seconds=-1670000000)
>>> t1 - t2
datetime.timedelta(0, 3600)

We lost an hour durint the 1st World War :-)

Whereas my implementation using mktime() works:

-1670000000.0