[issue17267] datetime.time support for '+' and '-'

[Martin Panter]

Martin Panter  added the comment:

A real use case that I have had was with a protocol to activate a device with a 
daily schedule. The protocol takes start and end hours and minutes of the day. 
To test the device by activating it over the next few minutes, my ideal way 
would have taken the current time (according to the device controller) as a 
“time” object, and added a couple of minutes using “timedelta”. In the end I 
think I made my protocol API accept both “time” and “timedelta" objects, 
because I found “timedelta” more flexible for calculations, but the “time” 
class more natural in other cases.

The start and end times are local times, and daylight saving could come into 
play, but in reality I won’t be testing the device at 3 a.m. on a Sunday 
morning. If I did care, I would have to add my own logic with knowledge of the 
date and daylight saving, to raise an exception.

I agree with Alexander about supporting the difference between two “time” 
instances. The result should be a non-negative “timedelta”, at least zero, and 
strictly less than 24 h.

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[issue17267] datetime.time support for '+' and '-'

[Andreas Åkerlund]

Change by Andreas Åkerlund :


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[issue17267] datetime.time support for '+' and '-'

[Skip Montanaro]

Change by Skip Montanaro :


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[issue17267] datetime.time support for '+' and '-'

[Paul Ganssle]

Paul Ganssle  added the comment:

I am pretty neutral on this. I don't think it will be terribly difficult to 
implement or maintain this, and while there are a few possible behaviors, if 
you think about it for a bit, addition with overflow behavior *does* seem like 
the natural way to implement it.

That said, I don't see an amazingly compelling use case for this. It's fairly 
rare to need to represent abstract times *at all*, and it's even more rare for 
performing arithmetic on those abstract times to be meaningful. I think the 
most dangerous aspect of this is that we might make it easier to do something 
that, for most people, would be the wrong thing to do.

Does anyone have some examples of real-world use cases for this, so that we're 
not designing in a vacuum?

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[issue17267] datetime.time support for '+' and '-'

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue17267] datetime.time support for '+' and '-'

[Cheryl Sabella]

Cheryl Sabella  added the comment:

It seems that there was interest in this enhancement a few years ago.  @joar, 
would you be able to convert your patch to a GitHub pull request on the master 
branch?  Thanks!

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versions: +Python 3.8 -Python 3.4

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[issue17267] datetime.time support for '+' and '-'

[R. David Murray]

Changes by R. David Murray rdmur...@bitdance.com:


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[issue17267] datetime.time support for '+' and '-'

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

I think the timezone related problems are a red herring.  Aware datetime +/- 
timedelta arithmetics is naive - tzinfo is ignored in calculations and copied 
to the result:

http://hg.python.org/cpython/file/c83ce2a1841c/Lib/datetime.py#l1711

The utcoffset only will only come into play if we want to implement time - time 
- timedelta, but this problem is already there in time comparisons:

http://hg.python.org/cpython/file/c83ce2a1841c/Lib/datetime.py#l1091

It is up to tzinfo subclass implementation writers to handle inability to 
compute utcoffset without date fields by raising an exception if necessary.  It 
is perfectly fine for time - time to fail with an error coming from 
.utcoffset().

I also don't think the fate of #13936 has any bearing on this issue.  As long 
as we are not trying to implement time + time - time, we are not introducing 
any new notion of zero time.

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[issue17267] datetime.time support for '+' and 'now'

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

I left a few minor comments on rietveld for the last patch.  I did not see code 
for time.now() and I don't think adding now() should be combined with time +/- 
timedelta patch.  Let's do one thing at a time.

I think time + timedelta addition is fairly uncontroversial.  In the past, I 
argued against using detached time objects, but it is not really a valid reason 
for rejecting a good feature.

On subtraction, if we add time - timedelta - time, I think users would expect 
time - time - timedelta as well.  This, however, is ambiguous if we stay with 
mod 24h arithmetic.  The ambiguity can be lifted by requiring days=0 in the 
result.

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[issue17267] datetime.time support for '+' and 'now'

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

This was proposed before and rejected in issue1118748, but I think current 
proposal addresses the ambiguity that was sited as a reason for rejection.

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[issue17267] datetime.time support for '+' and 'now'

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

See also issue 3250.  If we add mod 24h arithmetics, I would like to see 
something like time.add_with_carry(timedelta) - (int, time) method.  With it, 
users who need a specific overflow behavior will be able to implement it easily:

def check_add(t, td):
carry, result = t.add_with_carry(tf)
if carry:
raise ...

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[issue17267] datetime.time support for '+' and '-'

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

I am changing the title to focus this issue on arithmetics. Lack of time.now() 
is adressed in #8902.

--
title: datetime.time support for '+' and 'now' - datetime.time support for '+' 
and '-'

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[issue17267] datetime.time support for '+' and 'now'

[Francisco Martín Brugué]

Francisco Martín Brugué added the comment:

Hi Joar,
just a detail: is there a reason for the asymmetric check for timedelta 
isinstance (and raising NotImplemented)? And BTW. isn't a double check for the 
__sub__ case (or have I missed something)?

+def __add__(self, other):
+Add a time and a timedelta
+if not isinstance(other, timedelta):
+return NotImplemented

vs.
…
+def __sub__(self, other):
+Subtract a time and a timedelta.
+if isinstance(other, timedelta):
+return self + -other
+return NotImplemented

regards,
francis

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[issue17267] datetime.time support for '+' and 'now'

[jbatista]

jbatista added the comment:

IMHO this should be safe when the timezone is UTC for example, where there is 
no problems with daylight savings. What should be the behavior when adding a 
certain timedelta() and it crosses a date where there is an hour switch due to 
daylight savings? The unadvised would get incorrect results.

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[issue17267] datetime.time support for '+' and 'now'

[Ronald Oussoren]

Ronald Oussoren added the comment:

datetime.time arithmetic cannot be timezone aware, as there is no associated 
date and hence you cannot possibly know if there it a DST transition.

I don't think this is a problem. Adding/removing time to a clock value has 
clear real-world semantics. Using the (naive) real world semantics is the best 
we can do and should generally give the expected answer.

As to cross-timezone comparisons: 

   time(0, tzinfo=est) - timedelta(hours=1) * 5 == time(0, tzinfo=utc)

fails because the LHS of '==' is a time in a different timezone than the value 
on the RHS. That's expected and correct.

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[issue17267] datetime.time support for '+' and 'now'

[Petri Lehtinen]

Petri Lehtinen added the comment:

A time object isn't associated with any date, so I don't really see a
problem here. The fact that you can shoot yourself in the leg can be
documented, noting that you should use datetime instead.

ISTM the reason why time objects even have an associated timezone is
to support easy calculations between times in different timezones.

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[issue17267] datetime.time support for '+' and 'now'

[Joar Wandborg]

Joar Wandborg added the comment:

New patch, removed whitespace change and unnecessary test.

add_time_timedelta arg `factor` will not be changed, in order to preserve 
uniformity.

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Added file: http://bugs.python.org/file29251/issue17267-v3.patch

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[issue17267] datetime.time support for '+' and 'now'

[Petri Lehtinen]

Petri Lehtinen added the comment:

LGTM.

--
stage: test needed - patch review

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[issue17267] datetime.time support for '+' and 'now'

[Ronald Oussoren]

Changes by Ronald Oussoren ronaldousso...@mac.com:


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nosy: +belopolsky

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[issue17267] datetime.time support for '+' and 'now'

[Joar Wandborg]

Joar Wandborg added the comment:

I am adding yet another patch. This time it has

- TZ-aware tests (datetimetester.TestTimeTZ).
- C time_subtract method.
- Pure time.__sub__ method.

in addition to the previous

- Tests for time + timedelta
- C time_add
- Pure time.__add__

There's one issue though, and that is that I have not quite figured out 
TZ-aware cross-TZ `time - timedelta` i.e.

time(0, tzinfo=est) - timedelta(hours=1) * 5 == time(0, tzinfo=utc)

fails.

I have included a failing test for it.

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[issue17267] datetime.time support for '+' and 'now'

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


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[issue17267] datetime.time support for '+' and 'now'

[Joar Wandborg]

Joar Wandborg added the comment:

Patch submitted, please review.

--
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Added file: http://bugs.python.org/file29200/issue17267.patch

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[issue17267] datetime.time support for '+' and 'now'

[Petri Lehtinen]

Petri Lehtinen added the comment:

Added some comments to Rietveld.

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[issue17267] datetime.time support for '+' and 'now'

[Joar Wandborg]

Joar Wandborg added the comment:

New patch adressing the review comments.

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[issue17267] datetime.time support for '+' and 'now'

[Andreas Åkerlund]

Andreas Åkerlund added the comment:

Well I have also made a patch for this, using the datetime operator code as 
much as possible.. this is for version 3.4..

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Added file: http://bugs.python.org/file29206/issue17267-3.4.diff

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[issue17267] datetime.time support for '+' and 'now'

[Ronald Oussoren]

New submission from Ronald Oussoren:

It would be nice if datetime.time would be possible to add a delta to a 
datetime.time object, and if datetime.time had a method for returning the 
current time (just like datetime.date and date time.datetime have 'today' and 
'now' methods).

Rationale for the '+' operator: calculating the wall clock time some time after 
an time on an unspecified date. The easy solution would be:

   tm = datetime.time(13, 20)
   later = tm + datetime.timedelta(hours=5, minutes=44)

That's is currently not possible, but requires more complicated code.

Getting the current time without date information currently requires getting 
done with 'datetime.datetime.now().time()', a class method of 'datetime.time' 
would IMHO be nicer. I must admit that I don't have a good suggestion for the 
name of that method.

--
components: Library (Lib)
messages: 182592
nosy: ronaldoussoren
priority: normal
severity: normal
stage: test needed
status: open
title: datetime.time support for '+' and 'now'
type: enhancement
versions: Python 3.4

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[issue17267] datetime.time support for '+' and 'now'

[Eric V. Smith]

Eric V. Smith added the comment:

What would this give:

   tm = datetime.time(13, 20)
   later = tm + datetime.timedelta(hours=47, minutes=44)

datetime.time(13, 4)? Or raise an exception?

I've thought about this before, but it's always a problem when going over date 
boundaries. If you define + to be modulo 24 hours, then it's not very useful 
for cases I've looked at. Every time I've used time by itself, I end up going 
back to datetime. But I'll admit that might be a shortcoming of mine, not the 
concept.

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[issue17267] datetime.time support for '+' and 'now'

[Ronald Oussoren]

Ronald Oussoren added the comment:

IMHO this would to module 24 arithmetic, just like a normal clock.

When I do calculations with plain time that is what I want, if the date is also 
important I use datetime.datetime.  That a time value silently truncates when 
going past midnight is IMHO also the obvious behavior.

The biggest argument against adding this functionality I could come up with is 
that this can give wrong answers when daylight savings time transitions happen, 
which could lead to subtle bugs.

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