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handle DST appropriately in Timestamp.replace #18618

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Merged
merged 16 commits into from
Jan 5, 2018
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handle DST appropriately in Timestamp.replace #18618

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cfb760c
handle DST appropriately in Timestamp.replace
jbrockmendel Dec 4, 2017
4ca60fd
better comment, move test to test_timezones
jbrockmendel Dec 4, 2017
f738989
informative comment in test
jbrockmendel Dec 5, 2017
98a7296
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 6, 2017
6f36a97
handle DST appropriately in Timestamp.replace
jbrockmendel Dec 4, 2017
b916b17
better comment, move test to test_timezones
jbrockmendel Dec 4, 2017
b00d763
informative comment in test
jbrockmendel Dec 5, 2017
000f6cd
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 6, 2017
ec9b38a
Merge branch 'replace_bug' of https://github.com/jbrockmendel/pandas …
jbrockmendel Dec 6, 2017
f5886e2
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 7, 2017
4cfd949
fix duplicate whatsnews caused by merge mixups
jbrockmendel Dec 7, 2017
a3b14f9
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 14, 2017
504cf97
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 29, 2017
4459c9f
fixup merge messup
jbrockmendel Dec 29, 2017
1eb5f2f
Merge branch 'master' of https://github.com/pandas-dev/pandas into re…
jbrockmendel Dec 30, 2017
dced361
refactor test to apply to both pytz and dateutil
jbrockmendel Jan 4, 2018
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.23.0.txt
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Expand Up @@ -378,3 +378,4 @@ Other
^^^^^

- Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
- :func:`Timestamp.replace` will now handle Daylight Savings transitions gracefully (:issue:`18319`)
16 changes: 13 additions & 3 deletions pandas/_libs/tslibs/timestamps.pyx
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Expand Up @@ -33,7 +33,7 @@ from np_datetime cimport (reverse_ops, cmp_scalar, check_dts_bounds,
is_leapyear)
from timedeltas import Timedelta
from timedeltas cimport delta_to_nanoseconds
from timezones cimport get_timezone, is_utc, maybe_get_tz
from timezones cimport get_timezone, is_utc, maybe_get_tz, treat_tz_as_pytz

# ----------------------------------------------------------------------
# Constants
Expand Down Expand Up @@ -922,8 +922,18 @@ class Timestamp(_Timestamp):
_tzinfo = tzinfo

# reconstruct & check bounds
ts_input = datetime(dts.year, dts.month, dts.day, dts.hour, dts.min,
dts.sec, dts.us, tzinfo=_tzinfo)
if _tzinfo is not None and treat_tz_as_pytz(_tzinfo):
# replacing across a DST boundary may induce a new tzinfo object
# see GH#18319
ts_input = _tzinfo.localize(datetime(dts.year, dts.month, dts.day,
dts.hour, dts.min, dts.sec,
dts.us))
_tzinfo = ts_input.tzinfo
else:
ts_input = datetime(dts.year, dts.month, dts.day,
dts.hour, dts.min, dts.sec, dts.us,
tzinfo=_tzinfo)

ts = convert_datetime_to_tsobject(ts_input, _tzinfo)
value = ts.value + (dts.ps // 1000)
if value != NPY_NAT:
Expand Down
21 changes: 21 additions & 0 deletions pandas/tests/tseries/test_timezones.py
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Expand Up @@ -1228,6 +1228,27 @@ def f():
dt = Timestamp('2013-11-03 01:59:59.999999-0400', tz='US/Eastern')
assert dt.tz_localize(None) == dt.replace(tzinfo=None)

def test_replace_across_dst(self):
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can you tests with dateutil as well

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The test is pretty specific to pytz API

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The test is specific to the pytz API because the problem is specific to the pytz API, so you can't write tests that currently fail with dateutil, but you can, in fact, write this test to be agnostic as to whether the test takes a pytz or dateutil zone.

Rather than using localize and normalize, express the "expected" datetimes as UTC datetimes and use .astimezone. pytz and dateutil support astimezone (and, in fact, pytz.normalize essentially just uses .astimezone under the hood). Since pandas provides their own API layer on top of datetime, it is also agnostic to the timezone provider.

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Rather than using localize and normalize, express the "expected" datetimes as UTC datetimes and use .astimezone

Is this a suggestion for this test or more generally?

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Is this a suggestion for this test or more generally?

In this test at least, since with tests you can always engineer your datetime literals such that you never have to use anything except astimezone.

It's also just a useful piece of information to have, that astimezone is one of the few timezone functions where pytz behaves more or less nicely. You only need normalize semantics with pytz for things like replace or "calendar" arithmetic, where you want to change the naive portion of the datetime to a specific value and then change the time zone offset to match. If you want "absolute" arithmetic (where you want to go forward a certain number of hours or seconds or something), then the_operation(dt.astimezone(UTC)).astimezone(dt.tzinfo) will always give the right answer.

In this case you know what the correct answer should be, in absolute time, so you can just declare your initial variable as the correct answer in UTC and convert that to the time zone you care about. If you do it generically like that, you are insulated from any weird quirks of pytz's interface, and you get dateutil support for free (so you can parametrize this test using pytz and dateutil zones).

Of course, this method will fail if you try to construct an imaginary time, since there is no mapping between UTC and imaginary times.

# GH#18319 check that 1) timezone is correctly normalized and
# 2) that hour is not incorrectly changed by this normalization
tz = pytz.timezone('US/Eastern')

ts_naive = Timestamp('2017-12-03 16:03:30')
ts_aware = tz.localize(ts_naive)

# Preliminary sanity-check
assert ts_aware == ts_aware.tzinfo.normalize(ts_aware)

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need to parametrize this across dateutil & pytz, so you need to move this to TestTimeZoneSupportPytz and use self.tz(...)

# Replace across DST boundary
ts2 = ts_aware.replace(month=6)

# Check that `replace` preserves hour literal
assert (ts2.hour, ts2.minute) == (ts_aware.hour, ts_aware.minute)

# Check that post-replace object is appropriately normalized
ts2b = ts2.tzinfo.normalize(ts2)
assert ts2 == ts2b

def test_ambiguous_compat(self):
# validate that pytz and dateutil are compat for dst
# when the transition happens
Expand Down