datetime: define division timedelta/timedelta

@jribbens

BPO	2706
Nosy	@jribbens, @amauryfa, @mdickinson, @vstinner, @merwok, @bitdancer
Files	datetime_datetime_division.patch: patch as described datetime_datetime_division_dupcode.patch: patch without function pointers timedeltadiv.parch: Patch with tests against revision 67223 timedelta_true_divide_divmod.patch unnamed issue2706a.diff: patch against py3k revision 80210

^{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/mdickinson'
closed_at = <Date 2010-04-20.22:44:53.246>
created_at = <Date 2008-04-27.21:04:03.427>
labels = ['extension-modules', 'type-feature', 'library']
title = 'datetime: define division timedelta/timedelta'
updated_at = <Date 2010-04-21.17:54:08.177>
user = 'https://bugs.python.org/webograph'

bugs.python.org fields:

activity = <Date 2010-04-21.17:54:08.177>
actor = 'mark.dickinson'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2010-04-20.22:44:53.246>
closer = 'mark.dickinson'
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2008-04-27.21:04:03.427>
creator = 'webograph'
dependencies = []
files = ['10126', '10946', '12010', '12011', '16999', '17003']
hgrepos = []
issue_num = 2706
keywords = ['patch']
message_count = 39.0
messages = ['65902', '70018', '75876', '75877', '75878', '75880', '75881', '75882', '75883', '75884', '75892', '75894', '75895', '75908', '75909', '75913', '75917', '77646', '83416', '83447', '83453', '83936', '84131', '84151', '102956', '102962', '103615', '103631', '103634', '103635', '103636', '103637', '103638', '103653', '103664', '103787', '103788', '103867', '103872']
nosy_count = 11.0
nosy_names = ['jribbens', 'amaury.forgeotdarc', 'mark.dickinson', 'vstinner', 'jess.austin', 'fredrikj', 'webograph', 'eric.araujo', 'r.david.murray', 'tleeuwenburg@gmail.com', 'Alexander.Belopolsky']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue2706'
versions = ['Python 3.2']

i suggest that division be defined for timedelta1/timedelta2, in that
sense that it gives how many times timedelta2 fits in timedelta1 (ie the
usual meaning of division), using integer arithmetics for floor division
(//) and returning float for truediv (/ after from __future__ import division)

use case
--------

aside from the obvious how-many-times-does-a-fit-into-b, this solves the
issue of having individual methods for conversion to a number of
seconds, hours, days or nanocenturies (as described in bpo-1673409).
example:

from datetime import timedelta
duration = timedelta(hours=1.5, seconds=20)
print "Until the time is up, you can listen to 'We will rock you' %d
times."%(duration//timedelta(minutes=5, seconds=3))
import time
time.sleep(duration/timedelta(seconds=1))

history
-------

this issue follows a discussion on python-list, re-initiated by [1].

there have previously been similar feature requests on datetime, most of
which have been rejected due to ambiguities (e.g. [2]), conflicts with
time_t or issues with time zones.

the only issue i've seen that can be relevant here is the
integer-vs-float discussion, which is here handled by floordiv (//) and
truediv.

patch
-----

i've written a patch against svn trunk revision 62520.

it uses function pointers to reduce code duplication; in case this
inappropriate here, i also have a pointerless version.

i familiar with c but not experienced, especially with the python ways
of writing c. most of the code is just adapted from other functions in
the same files, so it is probably, but should nevertheless checked with
special care.

i've also added test, but am not sure what has to be tested and what not.

compatibility
-------------

only cases in which division would fail without the patch are changed.
this will be a problem if (and only if) someone divides unknown objects
and waits for TypeError to be raised.
such behavior is probably rare.

[1] <mid:4813CD56.40800@eml.cc>,
http://mail.python.org/pipermail/python-list/2008-April/488406.html
[2] http://mail.python.org/pipermail/python-dev/2002-March/020604.html

this is the mentioned patch without the function pointers, in case it
better fits the python coding style.

I attaching webograph's patch updated to revision 67223 where I added a
few tests.

I am +1 on the floor divide changes (allowing timedelta // timedelta),
but I am not sure how true division should work if at all. For the sake
of argument, let's assume from __future__ import division or py3k.
Currently:

>>> timedelta(1)/2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'datetime.timedelta' and 
'int'
>>> timedelta(1)/timedelta(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'datetime.timedelta' and 
'datetime.timedelta'

With this patch, timedelta(1)/2 is still unsupported, but

>>> timedelta(1)/timedelta(2)
0.5

Note that this is probably correct behavior because timedelta/int true
division does not make much sense, but there is a proposal (see bpo-1083) to make timedelta/int equivalent to timedelta//int. (I am
against bpo-1083 proposal and for webograph's approach, but this needs
to be further discussed.)

Also, I've added a test that demonstrates the following behavior:

>>> int(timedelta.min/(timedelta.min//3))
2

This is not a bug in webograph's patch, but rather a bug in long true
division, but it shows that true division may not be as useful as it
seems.

Why not also implementing divmod()? It's useful to split a timedelta
into, for example, (hours, minutes, seconds):

def formatTimedelta(delta):
    """
    >>> formatTimedelta(timedelta(hours=1, minutes=24, seconds=19))
    '1h 24min 19sec'
    """
    hours, minutes = divmodTimedelta(delta, timedelta(hours=1))
    minutes, seconds = divmodTimedelta(minutes, timedelta(minutes=1))
    seconds, fraction = divmodTimedelta(seconds, timedelta(seconds=1))
    return "{0}h {1}min {2}sec".format(hours, minutes, seconds)

My implementation gives divmod(timedelta, timedelta) -> (long,
timedelta). It's a little bit strange to get two different types in
the result. The second return value is the remainder. My example works
in the reverse order of the classical code:

def formatSeconds(seconds):
    """
    >>> formatTimedelta(1*3600 + 24*60 + 19)
    '1h 24min 19sec'
    """
    minutes, seconds = divmod(seconds, 60)
    hours, minutes = divmod(minutes, 60)
    return "{0}h {1}min {2}sec".format(hours, minutes, seconds)
    
About my new patch:
 - based on datetime_datetime_division_dupcode.patch
 - create divmod() operation on (timedelta, timedelta)
 - add unit tests for the division (floor and true division) and 
divmod
 - update the documentation for the true division and divmod

On Fri, Nov 14, 2008 at 12:51 PM, STINNER Victor <report@bugs.python.org> wrote:

STINNER Victor <victor.stinner@haypocalc.com> added the comment:

Why not also implementing divmod()? It's useful to split a timedelta
into, for example, (hours, minutes, seconds):

I agree and in this case mod should probably be implemented too.

With your patch:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for %: 'datetime.timedelta' and
'datetime.timedelta'

Also, why not

>>> divmod(timedelta(3), 2)
(datetime.timedelta(1), datetime.timedelta(1))

?

And where do we stop? :-)

On Fri, Nov 14, 2008 at 1:02 PM, Alexander Belopolsky
<belopolsky@users.sourceforge.net> wrote:
> On Fri, Nov 14, 2008 at 12:51 PM, STINNER Victor 
<report@bugs.python.org> wrote:
>>
>> STINNER Victor <victor.stinner@haypocalc.com> added the comment:
>>
>> Why not also implementing divmod()? It's useful to split a timedelta
>> into, for example, (hours, minutes, seconds):
>
> I agree and in this case mod should probably be implemented too.
>
> With your patch:
>
>>>> timedelta(3)%timedelta(2)
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> TypeError: unsupported operand type(s) for %: 'datetime.timedelta' and
> 'datetime.timedelta'
>

Since timedelta(3) // 2 is already accepted, divmod should also accept
integers (but not float).

With the last patch and "from __future__ import division", we support:
timedelta // <timedelta or int>
timedelta / timedelta
divmod(timedelta, timedelta)

What do you think about:
timedelta / <timedelta or int or float> # only with __future__.divison
timedelta // <timedelta or int>
timedelta % <timedelta or int>
divmod(timedelta, <timedelta or int>)
with:
timedelta // int -> timedelta
timedelta // timedelta -> int
timedelta % int -> timedelta
timedelta % timedelta -> int
divmod(timedelta, int) -> (timedelta, timedelta)
divmod(timedelta, timedelta) -> (int, timedelta)
timedelta / <anything> -> float # __future__.divison

While I agree that divmod may be useful, your particular use case is
not convincing. The same can be done easier without divmod:

def formatTimedelta(delta):
   return "{0}h {1}min {2}sec".format(*str(delta).split(':'))

or you can convert delta to time using an arbitrary anchor date and
extract hms that way:

(1, 24, 19)

(depending on your needs you may want to add delta.days*24 to the hours)

On Fri, Nov 14, 2008 at 12:51 PM, STINNER Victor <report@bugs.python.org> wrote:
>
> STINNER Victor <victor.stinner@haypocalc.com> added the comment:
>
> Why not also implementing divmod()? It's useful to split a timedelta
> into, for example, (hours, minutes, seconds):
>
> def formatTimedelta(delta):
>    """
>    >>> formatTimedelta(timedelta(hours=1, minutes=24, seconds=19))
>    '1h 24min 19sec'
>    """
>    hours, minutes = divmodTimedelta(delta, timedelta(hours=1))
>    minutes, seconds = divmodTimedelta(minutes, timedelta(minutes=1))
>    seconds, fraction = divmodTimedelta(seconds, timedelta(seconds=1))
>    return "{0}h {1}min {2}sec".format(hours, minutes, seconds)
>

On Fri, Nov 14, 2008 at 1:28 PM, STINNER Victor <report@bugs.python.org> wrote:
..

What do you think about:
timedelta / <timedelta or int or float> # only with __future__.divison
timedelta // <timedelta or int>
timedelta % <timedelta or int>
divmod(timedelta, <timedelta or int>)
with:
timedelta // int -> timedelta
already there

+1

timedelta % float -> timedelta (because int % float -> int works) ?

+1

divmod(timedelta, float) -> (timedelta, timedelta) ?

-1

Only timedelta / timedelta should produce dimensionless numbers.
timedelta / <float or int> should be disallowed in true division mode.
I am +0 on timedelta / timedelta -> float in true division mode.

Oops, the tracker ate some lines from e-mail. Reposting through the
web:

On Fri, Nov 14, 2008 at 1:28 PM, STINNER Victor <report@bugs.python.org>
wrote:
..

What do you think about:
timedelta / <timedelta or int or float> # only with
__future__.divison
timedelta // <timedelta or int>
timedelta % <timedelta or int>
divmod(timedelta, <timedelta or int>)
with:
timedelta // int -> timedelta
already there

timedelta // timedelta -> int
+1

timedelta % int -> timedelta
+1

timedelta % timedelta -> int
+1

timedelta % float -> timedelta (because int % float -> int works) ?

divmod(timedelta, int) -> (timedelta, timedelta)
+1

divmod(timedelta, timedelta) -> (int, timedelta)
+1

divmod(timedelta, float) -> (timedelta, timedelta) ?

timedelta / <anything> -> float # __future__.divison
-1

Only timedelta / timedelta should produce dimensionless numbers.
timedelta / <float or int> should be disallowed in true division mode.
I am +0 on timedelta / timedelta -> float in true division mode.
Reply

Forward

def formatTimedelta(delta):
return "{0}h {1}min {2}sec".format(*str(delta).split(':'))

OMG, this is ugly! Conversion to string and reparse the formatted text :-/
Your code doesn't work with different units than hours, minutes or seconds:

['4 days, 1', '32', '01']
>>> str(timedelta(hours=1, minutes=32, seconds=1, microseconds=2)).split(":")
['1', '32', '01.000002']

or you can convert delta to time using an arbitrary anchor date
and extract hms that way:

How? I don't understand your suggestion.

(depending on your needs you may want to add delta.days*24 to the hours)

The goal of the new operators (timedelta / timedelta, divmod(timedelta,
timedelta), etc.) is to avoid the use of the timedelta "internals" (days,
seconds and microseconds attributes) and give a new "natural" way to process
time deltas.

haypo> How? I don't understand your suggestion.

Sorry, another case of mail to tracker bug. Here is what I wrote:

"""
.. you can convert delta to time using an arbitrary anchor date and
extract hms that way:

>>> x = datetime(1,1,1) + timedelta(hours=1, minutes=24, seconds=19)
>>> x.hour,x.minute,x.second
(1, 24, 19)

(depending on your needs you may want to add delta.days*24 to the hours)
"""

but tracker ate the '>>>' lines :-(

@WeboGraph

@WeboGraph: time_gmtime() and time_localtime() already use function
pointer. I prefer function pointer than code duplication!

By the way, the patch looks good to me, as far as it goes, and I'm +1 on adding all this. I only have the tiniest of nits:

the patch deletes a line at the top of Lib/test/test_datetime.py, for no apparent reason
in delta_add, I think the added 'if' block should be an 'else if' (yes, I know this is excessively picky ;)
the brace style for if blocks is inconsistent: please put the opening brace on the same line as the if.

One other thought: with this division operation added, it might be nice to add constants like td_hour, td_minute, etc. to the module. Then the perennial 'how can I convert my timedelta x to minutes' questions could be answered with 'just do x/td_minute'. I would personally find x/td_second to be a more obvious and natural way to find the total number of seconds in a timedelta than x.total_seconds. I also quite like the idea of being able to create a 2.5-hour timedelta with something like

2*td_hour + 30*td_minute

On the other hand, maybe such constants would just be added clutter, since it's easy to create them when needed.

Tennessee, are you still tracking this issue? If not, can I steal it from you. :)

I found out that timedelta % int is not supported in the
released versions while timedelta // int is.

Mmm. Interesting. :)

I think it would be fine to add timedelta % timedelta in this patch, and then open another feature request for timedelta % int and friends as you suggest. I don't think divmod(timedelta, timedelta) should go in if timedelta % timedelta doesn't also go in.

I feel that increasing the number of added
features will reduce the likelihood of patch being accepted.

Perhaps; I haven't seen much opposition to these ideas anyway---I think the only reason they haven't been implemented yet is lack of round-tuits.

I'd be +1 on accepting the current patch if timedelta % timedelta were added to it.

Hmm. Having timedelta // int work is *really* peculiar, since it can only be made sense of with reference to some implicit particular chosen unit of time; in this case, that unit of time is apparently microseconds, as far as I can tell.

Surely there aren't any applications for timedelta // int?! The operation just doesn't make dimensional sense, since it involves taking floor of a timedelta.

I vote -3.2 minutes on extending this craziness by adding timedelta % int or divmod(timedelta, int).

I should be able to add timedelta % timedelta and fix the nits that Mark mentioned tonight.

On Tue, Apr 20, 2010 at 6:46 AM, Mark Dickinson <report@bugs.python.org>wrote:

Mark Dickinson <dickinsm@gmail.com> added the comment:

Tennessee, are you still tracking this issue? If not, can I steal it from
you. :)

Hi Mark,

Please feel free to steal it from me! Wonderful to see some movement on this
issue...

> I found out that timedelta % int is not supported in the
> released versions while timedelta // int is.

Mmm. Interesting. :)

I think it would be fine to add timedelta % timedelta in this patch, and
then open another feature request for timedelta % int and friends as you
suggest. I don't think divmod(timedelta, timedelta) should go in if
timedelta % timedelta doesn't also go in.

I'm an incrementalist -- I'm happy with one step at a time, so for what it's
worth I'd be happy to see divmod go in alone with % to be added later.
However, seeing as I'm really contributing, I'd say feel free to disregard
my vote of +1... :)

Regards,
-Tennessee

New patch, issue2706a.diff, implements timedelta % timedelta and addresses Mark's code review comments.

With respect to Mark's
"""
One other thought: with this division operation added, it might be nice to add constants like td_hour, td_minute, etc. to the module. Then the perennial 'how can I convert my timedelta x to minutes' questions could be answered with 'just do x/td_minute'. I would personally find x/td_second to be a more obvious and natural way to find the total number of seconds in a timedelta than x.total_seconds. I also quite like the idea of being able to create a 2.5-hour timedelta with something like

2*td_hour + 30*td_minute

On the other hand, maybe such constants would just be added clutter, since it's easy to create them when needed.
"""

I dislike this proposal for the same reason as Mark likes it: 2*td_hour + 30*td_minute == timedelta(hours=2, minutes=30) is a violation of TOOWTDI.

Stealing from Tennessee...

Patch committed to py3k in r80290, r80291, with some minor tweaks and fixes:

rename delta_remailder to delta_remainder
fix memory leak in delta_remainder
add whatsnew entry for docs
note that timedelta // timedelta returns an integer in docs
add tests for raising ZeroDivisionError, and remove uses of
lambda in favour of using the operator module

Thanks, Alexander!

Grr. s/whatsnew/versionadded/

Can someone (Mark?) add an acknowledgment for Victor Stinner to the NEWS file? My py3k patch was 90% code from Victor's trunk patch.

Ah, yes. Sorry, Victor! There's actually no acknowledgement in Misc/NEWS: it's not *that* common to put acknowledgements there, and I'm not sure it's necessary here, but I've fixed the commit message.

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datetime: define division timedelta/timedelta #46958

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datetime: define division timedelta/timedelta #46958

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webograph commented on Apr 27, 2008

webograph commented on Jul 19, 2008

abalkin commented on Nov 14, 2008

vstinner commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

vstinner commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

vstinner commented on Nov 14, 2008

abalkin commented on Nov 14, 2008

vstinner commented on Nov 14, 2008

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mdickinson commented on Apr 19, 2010

mdickinson commented on Apr 19, 2010

mdickinson commented on Apr 19, 2010

AlexanderBelopolsky commented on Apr 19, 2010

tleeuwenburggmailcom commented on Apr 19, 2010

AlexanderBelopolsky commented on Apr 20, 2010

mdickinson commented on Apr 20, 2010

mdickinson commented on Apr 20, 2010

AlexanderBelopolsky commented on Apr 21, 2010

mdickinson commented on Apr 21, 2010

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