Adding/subtracting when there's a change on DST #59

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1. sdispater commented on Oct 22, 2016

   

   sdispater

   on Oct 22, 2016

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   You are right, this is (well was 😃 ) an undesired behavior.

   I just pushed a fix (660c6b9) on the master branch.

   Basically, pendulum will now treat years, months, weeks and days as whole units and will not apply the transition but only set the proper timezone. Even if you combine it with other units (hours, minutes, seconds and microseconds).

   Let's take an example:

   import pendulum
   
   dt = pendulum.create(2016, 2, 20, 22, tz='America/Sao_Paulo')
   # <Pendulum [2016-02-20T22:00:00-02:00]>
   
   # Let's add 3 hours (we will stumble across the DST transition)
   dt.add(hours=3)
   # <Pendulum [2016-02-21T00:00:00-03:00]>
   
   # Let's add 1 day
   # (the transition has occurred but we won't apply it (i.e. remove one hour))
   dt.add(days=1)
   # <Pendulum [2016-02-21T22:00:00-03:00]>
   
   # But the difference will actually be 25 hours
   dt.add(days=1).diff(dt).in_hours()
   # 25

   As for your initial example:

   >>> pendulum.parse("2016-01-12", tz="America/Sao_Paulo").add(months=4)
   # <Pendulum [2016-05-12T00:00:00-03:00]>
   
   >>> pendulum.parse("2016-01-30", tz="America/Sao_Paulo").add(months=4)
   # <Pendulum [2016-05-30T00:00:00-03:00]>
2. danilobellini commented on Oct 22, 2016

   

   danilobellini

   on Oct 22, 2016

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   Blazingly fast! =)

   I installed the git tip to try that commit, and I've found something perhaps related to the date adding/subtracting (I'm using IPython):

   In [1]: import pendulum
   
   In [2]: # Change the string format just to avoid some undesired coloring on GitHub
   
   In [3]: pendulum.set_to_string_format("'%Y-%m-%d %H:%M:%S %_z'")
   
   In [4]: dt = pendulum.parse("2016-01-30", tz="America/Sao_Paulo")
   
   In [5]: p = dt.add(months=4) - dt
   
   In [6]: p.range("months")
   Out[6]: 
   [<Pendulum ['2016-01-30 00:00:00 -02:00']>,
    <Pendulum ['2016-02-29 00:00:00 -03:00']>,
    <Pendulum ['2016-03-29 00:00:00 -03:00']>,
    <Pendulum ['2016-04-29 00:00:00 -03:00']>,
    <Pendulum ['2016-05-29 00:00:00 -03:00']>]

   The period range seem to get the previous output to add, instead of using its p.start value. I was expecting the result above to be like this:

   In [7]: [dt.add(months=m) for m in range(5)]
   Out[7]: 
   [<Pendulum ['2016-01-30 00:00:00 -02:00']>,
    <Pendulum ['2016-02-29 00:00:00 -03:00']>,
    <Pendulum ['2016-03-30 00:00:00 -03:00']>,
    <Pendulum ['2016-04-30 00:00:00 -03:00']>,
    <Pendulum ['2016-05-30 00:00:00 -03:00']>]

   That shouldn't make any difference when adding time (hours, minutes, seconds, microseconds), but when we're adding dates (days, weeks, months, years) that matters a lot as some_dt_obj.add(days=1).add(days=1) and some_dt_obj.add(days=2) might give us different results. The aforementioned range example isn't about DST, but here's an example that propagates something specific to a DST transition:

   In [8]: my_birthday = pendulum.parse("2016-10-14", tz="America/Sao_Paulo")
   
   In [9]: (my_birthday.add(weeks=1) - my_birthday).range("days")
   Out[9]: 
   [<Pendulum ['2016-10-14 00:00:00 -03:00']>,
    <Pendulum ['2016-10-15 00:00:00 -03:00']>,
    <Pendulum ['2016-10-16 01:00:00 -02:00']>,
    <Pendulum ['2016-10-17 01:00:00 -02:00']>,
    <Pendulum ['2016-10-18 01:00:00 -02:00']>,
    <Pendulum ['2016-10-19 01:00:00 -02:00']>,
    <Pendulum ['2016-10-20 01:00:00 -02:00']>]
   
   In [10]: [my_birthday.add(days=d) for d in range(7)]
   Out[10]: 
   [<Pendulum ['2016-10-14 00:00:00 -03:00']>,
    <Pendulum ['2016-10-15 00:00:00 -03:00']>,
    <Pendulum ['2016-10-16 01:00:00 -02:00']>,
    <Pendulum ['2016-10-17 00:00:00 -02:00']>,
    <Pendulum ['2016-10-18 00:00:00 -02:00']>,
    <Pendulum ['2016-10-19 00:00:00 -02:00']>,
    <Pendulum ['2016-10-20 00:00:00 -02:00']>]
3. sdispater commented on Oct 22, 2016

   

   sdispater

   on Oct 22, 2016

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   For your first example, I agree that the behavior you expect should be the correct one since if you just do dt.add(months=4) you will get <Pendulum ['2016-05-30 00:00:00 -03:00']> but this is not the last instance returned by range(). 

   As for you second example, I think range() should return the second batch of results too.

   Anyway, the two are related anyway and it should be an easy fix. I will keep you posted about the resolution of this issue.
4. sdispater commented on Oct 22, 2016

   

   sdispater

   on Oct 22, 2016

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   It should be fixed on the master branch.
5. danilobellini commented on Oct 22, 2016

   

   danilobellini

   on Oct 22, 2016

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   Wow! Thanks! Now my birthday is in your test_range_with_dst code. ^^

   As some_dt.start_of("week") calls self.previous, which happens to call the subtract method, the other places where I had seen a similar behavior (namely, the next, previous, start_of and end_of methods) seem to be fixed together as they call the same add and subtract methods.

   I'll be using the git tip until a version bump with this fix gets available. =)
6. danilobellini

   closed this as completedon Oct 22, 2016