# xarray-einstats#

Stats, linear algebra and einops for xarray

⚠️ Caution: This project is still in an early development stage

## Installation#

To install, run

(.venv) \$ pip install xarray-einstats


## Overview#

As stated in their website:

xarray makes working with multi-dimensional labeled arrays simple, efficient and fun!

The code is often more verbose, but it is generally because it is clearer and thus less error prone and more intuitive. Here are some examples of such trade-off where we believe the increased clarity is worth the extra characters:

numpy

xarray

a[2, 5]

da.sel(drug="paracetamol", subject=5)

a.mean(axis=(0, 1))

da.mean(dim=("chain", "draw"))

a.reshape((-1, 10))

da.stack(sample=("chain", "draw"))

a.transpose(2, 0, 1)

da.transpose("drug", "chain", "draw")

In some other cases however, using xarray can result in overly verbose code that often also becomes less clear. xarray_einstats provides wrappers around some numpy and scipy functions (mostly numpy.linalg and scipy.stats) and around einops with an api and features adapted to xarray.

### Data for examples#

The examples in this overview page use the DataArrays from the Dataset below (stored as ds variable) to illustrate xarray_einstats features:

<xarray.Dataset>
Dimensions:  (dim_plot: 50, chain: 4, draw: 500, team: 6)
Coordinates:
* chain    (chain) int64 0 1 2 3
* draw     (draw) int64 0 1 2 3 4 5 6 7 8 ... 492 493 494 495 496 497 498 499
* team     (team) object 'Wales' 'France' 'Ireland' ... 'Italy' 'England'
Dimensions without coordinates: dim_plot
Data variables:
x_plot   (dim_plot) float64 0.0 0.2041 0.4082 0.6122 ... 9.592 9.796 10.0
atts     (chain, draw, team) float64 0.1063 -0.01913 ... -0.2911 0.2029
sd_att   (draw) float64 0.272 0.2685 0.2593 0.2612 ... 0.4112 0.2117 0.3401


### Stats#

xarray_einstats.stats provides two wrapper classes xarray_einstats.stats.XrContinuousRV and xarray_einstats.stats.XrDiscreteRV that can be used to wrap any distribution in scipy.stats so they accept DataArray as inputs, and some wrappers for other functions in the scipy.stats module so you can use dims (supporting both string and iterable of strings) instead of axis and keep the labels from the input DataArrays.

The distribution wrappers perform broadcasting and alignment of all the inputs automatically. You can evaluate the logpdf using inputs that wouldn’t align if using numpy in a couple lines:

norm_dist = xarray_einstats.stats.XrContinuousRV(scipy.stats.norm)
#   shapes:         (50,)     (4, 500, 6)     (500,)
norm_dist.logpdf(ds["x_plot"], ds["atts"], ds["sd_att"])


which returns:

<xarray.DataArray (dim_plot: 50, chain: 4, draw: 500, team: 6)>
array([[[[ 3.06470249e-01,  3.80373065e-01,  2.56575936e-01,
...
-4.41658154e+02, -4.57599982e+02, -4.14709280e+02]]]])
Coordinates:
* chain    (chain) int64 0 1 2 3
* draw     (draw) int64 0 1 2 3 4 5 6 7 8 ... 492 493 494 495 496 497 498 499
* team     (team) object 'Wales' 'France' 'Ireland' ... 'Italy' 'England'
Dimensions without coordinates: dim_plot


More examples available at Intro to the stats module.

### Linear Algebra#

There is no one size fits all solution, but knowing the function we are wrapping we can easily make the code more concise and clear. Without xarray_einstats, to invert a batch of matrices stored in a 4d array you have to do:

inv = xarray.apply_ufunc(   # output is a 4d labeled array
numpy.linalg.inv,
batch_of_matrices,      # input is a 4d labeled array
input_core_dims=[["matrix_dim", "matrix_dim_bis"]],
output_core_dims=[["matrix_dim", "matrix_dim_bis"]]
)


to calculate it’s norm instead, it becomes:

norm = xarray.apply_ufunc(  # output is a 2d labeled array
numpy.linalg.norm,
batch_of_matrices,      # input is a 4d labeled array
input_core_dims=[["matrix_dim", "matrix_dim_bis"]],
)


With xarray_einstats.linalg, those operations become:

inv = xarray_einstats.inv(batch_of_matrices, dim=("matrix_dim", "matrix_dim_bis"))
norm = xarray_einstats.norm(batch_of_matrices, dim=("matrix_dim", "matrix_dim_bis"))


Moreover, if you use some internal conventions to label the dimensions that correspond to matrices, so that they can always be identified if given the list of all dimensions in the input, you can configure xarray_einstats to follow that convention. Take a look at get_default_dims

And if you still need more reasons for xarray_einstats, to complement the einops wrappers, it also provides xarray_einstats.einsum!

More examples available, also using einsum at Intro to the linear algebra module.

### einops#

repeat wrapper still missing

einops uses a convenient notation inspired in Einstein notation to specify operations on multidimensional arrays. It uses spaces as a delimiter between dimensions, parenthesis to indicate splitting or stacking of dimensions and -> to separate between input and output dim specification.

xarray_einstats.einops uses an adapted notation to take advantage of xarray, where dimensions are already labeled, and adapts to dimension names with spaces or parenthesis in them. It then translates the expression and calls einops via xarray.apply_ufunc so you need to have einops installed for the functions in this module to work.

xarray_einstats uses two separate arguments, one for the input pattern (optional) and another for the output pattern. Each is a list of dimensions (strings) or dimension (lists or dictionaries).

Tip

If you are willing to impose some extra constraints to your dimension names, you can also use the raw_ einops wrappers, with a syntax more concise and much closer to the einops library.

Combine the chain and draw dimensions

We can combine the chain and draw dimensions and name the resulting dimension sample using a list with a single dictionary.

rearrange(ds.atts, [{"sample": ("chain", "draw")}])


As you would do in einops, we indicate we want to combine the chain and draw dimensions by putting the two inside a parenthesis. With xarray_einstats in addition, you can add an =new_name to label this combined dimension, otherwise it gets a default name.

Moreover, as dimensions are already labeled in the input, we can skip the left side of the expression. If no -> symbol is present in the pattern, xarray_einstats generates the left side automatically.

raw_rearrange(ds.atts, "(chain draw)=sample")


The team dimension is not present in the pattern and is not modified. As here dimensions are named already in the input object, we don’t need ellipsis nor adding dimensions in both input and output to indicate they are left as is. You can see how the team dimension has not been modified in the output below:

<xarray.DataArray 'atts' (team: 6, sample: 2000)>
array([[ 0.10632395,  0.1538294 ,  0.17806237, ...,  0.16744257,
0.14927569,  0.21803568],
...,
[ 0.30447644,  0.22650416,  0.25523419, ...,  0.28405435,
0.29232681,  0.20286656]])
Coordinates:
* team     (team) object 'Wales' 'France' 'Ireland' ... 'Italy' 'England'
Dimensions without coordinates: sample


Note that following xarray convention, new dimensions and dimensions on which we operated are moved to the end. This only matters when you access the underlying array with .values or .data and you can always transpose using xarray.Dataset.transpose, but it can matter. You can change the pattern to enforce the output dimension order:

rearrange(ds.atts, [{"sample": ("chain", "draw")}, "team"])

raw_rearrange(ds.atts, "(chain draw)=sample team")


Out:

<xarray.DataArray 'atts' (sample: 2000, team: 6)>
array([[ 0.10632395, -0.01912607,  0.13671159, -0.06754783, -0.46083807,
0.30447644],
...,
[ 0.21803568, -0.11394285,  0.09447937, -0.11032643, -0.29111234,
0.20286656]])
Coordinates:
* team     (team) object 'Wales' 'France' 'Ireland' ... 'Italy' 'England'
Dimensions without coordinates: sample


Decompose and combine two dimensions in a different order

Now to a more complicated pattern. We will split the chain and team dimension, then combine those split dimensions between them.

Use a list of dictionaries to choose which dimensions to decompose, note that lists with dimensions to decompose are not valid, you need to indicate which dimension is the one to be decomposed.

rearrange(
ds.atts,
in_dims=[{"chain": ("chain1", "chain2")}, {"team": ("team1", "team2")}],
# combine split chain and team dims between them
# here we don't use a dict so the new dimensions get a default name
out_dims=[("chain1", "team1"), ("team2", "chain2")],
# set the lengths of split dimensions as kwargs
chain1=2, chain2=2, team1=2, team2=3
)


We use ()= on the left side because we need to indicate which dimensions to decompose, but we can skip it if we want on the right side and xarray_einstats uses a default name for them.

raw_rearrange(
ds.atts,
"(chain1 chain2)=chain (team1 team2)=team -> (chain1 team1) (team2 chain2)",
# set the lengths of split dimensions as kwargs
chain1=2, chain2=2, team1=2, team2=3
)


Out:

<xarray.DataArray 'atts' (draw: 500, chain1,team1: 4, team2,chain2: 6)>
array([[[ 1.06323952e-01,  2.47005252e-01, -1.91260714e-02,
-2.55769582e-02,  1.36711590e-01,  1.23165119e-01],
...
[-2.76616968e-02, -1.10326428e-01, -3.99582340e-01,
-2.91112341e-01,  1.90714405e-01,  2.02866563e-01]]])
Coordinates:
* draw     (draw) int64 0 1 2 3 4 5 6 7 8 ... 492 493 494 495 496 497 498 499
Dimensions without coordinates: chain1,team1, team2,chain2


More einops examples with both rearrange and reduce at Einops tutorial (ported)

### Other features#

xarray_einstats also includes some functions that are not direct wrappers of other libraries. histogram for example combines numba, numpy and xarray to provide a vectorized version of numpy.histogram that works on DataArrays.

## Similar projects#

Here we list some similar projects we know of. Note that all of them are complementary and don’t overlap:

## Cite xarray-einstats#

If you use this software, please cite it using the following template and the version specific DOI provided by Zenodo. Click on the badge to go to the Zenodo page and select the DOI corresponding to the version you used

• Oriol Abril-Pla. (2022). arviz-devs/xarray-einstats <version>. Zenodo. <version_doi>

or in bibtex format:

@software{xarray_einstats2022,
author       = {Abril-Pla, Oriol},
title        = {{xarray-einstats}},
year         = 2022,
url          = {https://github.com/arviz-devs/xarray-einstats}
publisher    = {Zenodo},
version      = {<version>},
doi          = {<version_doi>},
}