SettingWithCopyWarning when setting datetime value in pandas Series

Question

I am getting a SettingWithCopyWarning when setting a value in a pandas Series using iloc. I am using pandas 0.21.1 and python 3.6.7

import pandas as pd
from datetime import datetime
from pytz import timezone

tz = timezone('CET')
ambiguous_dst = True

expected_dt_series = pd.Series([
    datetime(2018, 10, 28, 1),
    datetime(2018, 10, 28, 2),
    datetime(2018, 10, 28, 3),
    datetime(2018, 10, 28, 4),
])
expected_dt_series = expected_dt_series.dt.tz_localize(
    tz, ambiguous='NaT')

expected_dt_series.iloc[1] = tz.localize(
    datetime(2018, 10, 28, 2), is_dst=ambiguous_dst) # <- line that causes error

expected_dt_series = expected_dt_series.astype('object')

output:

SettingWithCopyWarning: modifications to a method of a datetimelike object are not supported and are discarded. Change values on the original.
  self._setitem_with_indexer(indexer, value)
  x.py:17: SettingWithCopyWarning: modifications to a method of a datetimelike object are not supported and are discarded. Change values on the original.
    datetime(2018, 10, 28, 2), is_dst=ambiguous_dst)

Why does this happen? I am setting a value in the original series
What can I do to avoid this? (Other than turning off the warning)

It_is_Chris · Accepted Answer · 2018-12-14 04:47:15Z

3

it is really this part that is causing the problem:

expected_dt_series = expected_dt_series.dt.tz_localize(tz, ambiguous='NaT')

explicitly tell pandas that this is its own series by using copy

expected_dt_series = expected_dt_series.dt.tz_localize(
    tz, ambiguous='NaT').copy()

expected_dt_series.iloc[1] = tz.localize(
    datetime(2018, 10, 28, 2), is_dst=ambiguous_dst)

edited Dec 14, 2018 at 4:47

answered Dec 14, 2018 at 4:08

It_is_Chris

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1 Comment

A.H Over a year ago

Got it thanks. I got mixed up between the documentation of Series.dt.tz_localize and Series.tz_localize.

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