python xarray – update subset of values

Question

I need to update values in a Xarray DataArray. There are only certain values I want to update, which are stored in a dataframe together with their coordinates.

One can also reformulate the problem as both (i) update a non-continuous (non-rectangular) area of values in a DataArray and (ii) left-join dataset with new values on matching coords and rewrite old values.

Intial DataArray (Dataset):

import xarray as xr
import pandas as pd

ds = xr.Dataset(
    data_vars = {"x": (("a", "b"), np.zeros((3, 4)))},
    coords = {"a": np.arange(3), "b": np.arange(4)})
ds.x.values    

array([[0., 0., 0., 0.],
      [0., 0., 0., 0.],
      [0., 0., 0., 0.]])

New values to assign with their coordinates:

new_val = pd.DataFrame([
    (0, 1, 10),
    (1, 0, 11),
    (2, 2, 12)],
    columns=['a', 'b', 'x'])

Result I want to get:

array([[0., 10., 0., 0.],
      [11., 0., 0., 0.],
      [0., 0., 12., 0.]])

I was trying to use methods both in Combining data and in Assigning values with indexing turials, but no luck so far.

Asked By: Pepacz

||

Source

Answer 1

Result can be achieved using combine_first.

# set index and convert to xr Dataset
new_val_idx = new_val.set_index(['a', 'b'])
new_val_ds = xr.Dataset.from_dataframe(new_val_idx)

combined = new_val_ds.combine_first(ds)
combined.x.values

array([[ 0., 10.,  0.,  0.],
       [11.,  0.,  0.,  0.],
       [ 0.,  0., 12.,  0.]])

Answered By: Pepacz

python xarray – update subset of values

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