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Keep index dimension when selecting only a single coord #3458

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ngreenwald opened this issue Oct 29, 2019 · 5 comments
Open

Keep index dimension when selecting only a single coord #3458

ngreenwald opened this issue Oct 29, 2019 · 5 comments
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@ngreenwald
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@ngreenwald ngreenwald commented Oct 29, 2019 •
edited

MCVE Code Sample

# Your code here
import numpy as np
import xarray as xr

data = np.zeros((10, 4))
example_xr = xr.DataArray(data, coords=[range(10), ["idx0", "idx1", "idx2", "dim3"]], dims=["rows", "cols"])

# desired behavior
subset = example_xr[:, 1:2]
subset.shape

# inclusive indexing means both idx1 and idx2 kept
subset_named1 = example_xr.loc[:, "idx1":"idx2"]
subset_named1.shape

# slicing behavior means that 2nd dimension is dropped
subset_named2 = example_xr.loc[:, "idx1"]
subset_named2.shape

Expected Output

I'd like to be able to use named .loc indexing to select only a single named coord from one dimension, but not have that dimension collapse when subsetting.

Problem Description

I looked, but wasn't able to find anything in the documentation about how to perform this same action using named coords. It works with integer-based slicing.

Output of xr.show_versions()

# Paste the output here xr.show_versions() here
@mathause
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@mathause mathause commented Oct 29, 2019

I think example_xr.loc[:, ["idx1"]] should do what you want.
@dcherian
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@dcherian dcherian commented Oct 29, 2019 •
edited

We should add this as an example in the documentation since it comes up often enough.
@ngreenwald
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@ngreenwald ngreenwald commented Oct 29, 2019

@max-sixty
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@max-sixty max-sixty commented Oct 29, 2019

Also example_xr.sel(cols=['idx1']) for the more idiomatic use of dim names
@NowanIlfideme
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@NowanIlfideme NowanIlfideme commented Nov 22, 2019

Note that this doesn't work on MultiIndex levels, since vectorized operations on them are not currently supported. Meanwhile, using sel(multiindex_level_name="a") drops the level from the multiindex entirely. The running theme is that this is dependent on #1603, it seems. :)
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@NowanIlfideme @dcherian @max-sixty @mathause @ngreenwald
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