xarray - apply a function to the time dimension of a DataArray and add the result as a variable

Question

I have a DataArray da with a variable called FFDI which has three dimensions, time , latitude and longitude .

xarray.DataArray   'FFDI'   time: 43848   latitude: 2   longitude: 244

latitude     (latitude)     float32           -39.2 -39.163948
longitude    (longitude)    float32           140.8 140.83786 ... 149.96214 150.0
time         (time)         datetime64[ns]    2000-01-01T00:00:00  2000-01-01T01:00:00  ...  2004-12-31T22:00:00  2004-12-31T23:00:00

What I want to achieve is to apply the following function to each timestamp of the time dimension to calculate if the timestamp is during the Daylight Savings period and the output a boolean.

def isDST(dt_str):
    local_time_tz = pytz.timezone("Australia/Victoria")
    naive_datetime = datetime.datetime.strptime (dt_str, "%Y-%m-%d %H:%M:%S")
    a = local_time_tz.localize(naive_datetime)
    return bool(a.dst())

The output would be an numpy array or another DataArray element; then it would be added to the original da as additional variable named isDST .

xarray.DataArray   'FFDI'   time: 43848   latitude: 2   longitude: 244

latitude     (latitude)     float32           -39.2 -39.163948
longitude    (longitude)    float32           140.8 140.83786 ... 149.96214 150.0
time         (time)         datetime64[ns]    2000-01-01T00:00:00  2000-01-01T01:00:00  ...  2004-12-31T22:00:00  2004-12-31T23:00:00
isDST        (time)         bool              true true ... true true

Is this possible and what function should be used in xarray, pandas or numpy?

Answer 1

You can just use a simple list comprehension and add the new coordinate to the DataArray :

import xarray as xr
import pytz

# example data
x = xr.tutorial.load_dataset("air_temperature").air

local_time_tz = pytz.timezone("Australia/Victoria")
is_dst = [bool(local_time_tz.localize(x).dst()) for x in x.time.to_index()]
x = x.assign_coords(is_dst=("time", is_dst))

# result
# Coordinates:
#   * lat      (lat) float32 75.0 72.5 70.0 67.5 65.0 ... 25.0 22.5 20.0 17.5 15.0
#   * lon      (lon) float32 200.0 202.5 205.0 207.5 ... 322.5 325.0 327.5 330.0
#   * time     (time) datetime64[ns] 2013-01-01 ... 2014-12-31T18:00:00
#     is_dst   (time) bool True True True True True ... True True True True True

If x were a xarray.Dataset instead of a xarray.DataArray you could simply do:

x['is_dst'] = [bool(local_time_tz.localize(x).dst()) for x in x.time.to_index()]