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Confusing terminologies and some errors in the official documentation #6866

Description

@v-liuwei

What happened?

To note, I'm using the stable version(2022.6.0).

First, I'm confused that both dimension coordinate/non-dimension coordinate and index coordinate/non-index coordinate appear in the documentation(search to see), but they seem to be the same thing.

Second, I found that there are some errors in the documentation:

  • It says that "The index associated with dimension name x can be retrieved by arr.indexes[x]. By construction, len(arr.dims) == len(arr.indexes)", which is inconsistent with actual behavior. See example code below:

    In [0]: import xarray as xr, numpy as np
    In [1]: arr = xr.DataArray(np.zeros((2, 3)), dims=['x', 'y'], coords={'x': ['a', 'b']})
    In [2]: assert len(arr.dims) == len(arr.indexes), f"{len(arr.dims)=}, {len(arr.indexes)=}"
    ---------------------------------------------------------------------------
    AssertionError                            Traceback (most recent call last)
    <ipython-input-202-f217d18e6979> in <module>
    ----> 1 assert len(arr.dims) == len(arr.indexes), f"{len(arr.dims)=}, {len(arr.indexes)=}"
    
    AssertionError: len(arr.dims)=2, len(arr.indexes)=1
    In [3]: arr.indexes
    Out[3]:
    Indexes:
    x: Index(['a', 'b'], dtype='object', name='x')

    It seems that arr.indexes only returns indexes of dimensions that have coordinates. However, it's possible to get the index of
    dimension y through get_index():

    In [4]: arr.get_index('y')
    Out[4]: RangeIndex(start=0, stop=3, step=1, name='y')
  • It says that: (see link)

    For convenience multi-index levels are directly accessible as “virtual” or “derived” coordinates (marked by - when printing a dataset or data array):

    In [77]: mda["band"]
    Out[77]: 
    <xarray.DataArray 'band' (spec: 4)>
    array(['R', 'R', 'V', 'V'], dtype=object)
    Coordinates:
      * spec     (spec) object MultiIndex
      * band     (spec) object 'R' 'R' 'V' 'V'
      * wn       (spec) float64 0.1 0.2 0.7 0.9
    
    In [78]: mda.wn
    Out[78]: 
    <xarray.DataArray 'wn' (spec: 4)>
    array([0.1, 0.2, 0.7, 0.9])
    Coordinates:
      * spec     (spec) object MultiIndex
      * band     (spec) object 'R' 'R' 'V' 'V'
      * wn       (spec) float64 0.1 0.2 0.7 0.9

    As you can see, even in the given example code offered by the offical, all the "virtual" coordinates are marked as * instead of -, which is a little bit confusing when handling multi-index coordinates in my experience.

May I have missed something? Thanks in advance for the reply.

What did you expect to happen?

No response

Minimal Complete Verifiable Example

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MVCE confirmation

  • Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
  • Complete example — the example is self-contained, including all data and the text of any traceback.
  • Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
  • New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

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Anything else we need to know?

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Environment

Details

INSTALLED VERSIONS

commit: None
python: 3.8.10 (default, Sep 28 2021, 16:10:42)
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 5.10.102.1-microsoft-standard-WSL2
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: None
libnetcdf: None

xarray: 2022.6.0
pandas: 1.4.3
numpy: 1.23.1
scipy: 1.3.3
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.1.2
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 45.2.0
pip: 22.2.1
conda: None
pytest: None
IPython: 7.13.0
sphinx: None

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