xarray-units

xarray extension for handling units

Overview

xarray-units is an import-only package that provides a xarray DataArray accessor .units for handling units such as converting units and numeric operations considering units. Astropy is used as a backend. Unlike similar implementations, xarray-units does not use a special data type to handle units, but uses the original data type in a DataArray. This allows to continue to use powerful features such as parallel and lazy processing with Dask and/or user-defined DataArray subclasses.

Installation

pip install xarray-units==0.5.0

Basic usages

Suppose the following imports will be commonly used in the examples:

import xarray as xr
import xarray_units

Setting and unsetting units

xarray-units sets units in DataArray attributes (.attrs) with the name "units":

da_km = xr.DataArray([1, 2, 3]).units.set("km")
print(da_km)

<xarray.DataArray (dim_0: 3)>
array([1, 2, 3])
Dimensions without coordinates: dim_0
Attributes:
    units:    km

And the units can also be unset (deleted):

da = da_km.units.unset()
print(da)

<xarray.DataArray (dim_0: 3)>
array([1, 2, 3])
Dimensions without coordinates: dim_0

[!NOTE] These are equivalent to manually un/setting the units in the DataArray attributes, but the units accessor also check that the units are valid when setting.

Converting units to others

xarray-units converts a DataArray with units to other units:

da_km = xr.DataArray([1, 2, 3]).units.set("km")
da_m = da_km.units.to("m")
print(da_m)

<xarray.DataArray (dim_0: 3)>
array([1000., 2000., 3000.])
Dimensions without coordinates: dim_0
Attributes:
    units:    m

Astropy equivalencies can also be used for equivalences between different units:

from astropy.units import spectral

da_mm = xr.DataArray([1, 2, 3]).units.set("mm")
da_GHz = da_mm.units.to("GHz", spectral())
print(da_GHz)

<xarray.DataArray (dim_1: 3)>
array([299.792458  , 149.896229  ,  99.93081933])
Dimensions without coordinates: dim_0
Attributes:
    units:    GHz

[!TIP] There exist other accessor methods (e.g. decompose, like) for converting units. See the package guide for more details.

Numeric operations considering units

xarray-units performs numerical operations considering units when the units accessor is attached to the DataArray on the left side of the operator:

da_m = xr.DataArray([1000, 2000, 3000]).units.set("m")
da_km = xr.DataArray([1, 2, 3]).units.set("km")

da_sum_m = da_m.units + da_km
da_sum_km = da_km.units + da_m

print(da_sum_m)
print(da_sum_km)

<xarray.DataArray (dim_0: 3)>
array([2000., 4000., 6000.])
Dimensions without coordinates: dim_0
Attributes:
    units:    m

<xarray.DataArray (dim_0: 3)>
array([2., 4., 6.])
Dimensions without coordinates: dim_0
Attributes:
    units:    km

The units of the DataArray after the operation follows those of the DataArray with the units accessor. The resulting data values will be therefore different depending on the order of the operation. They are, of course, equal when considering units:

da_eq = (da_sum_m.units == da_sum_km)
print(da_eq)

<xarray.DataArray (dim_0: 3)>
array([ True,  True,  True])
Dimensions without coordinates: dim_0

[!IMPORTANT] Because this feature is accessor-based, units are only considered for the operation right after the units accessor. See method and operation chains for performing multiple operations at once.

[!TIP] There exist accessor methods corresponding to each operator (e.g. add → +, eq → ==). See the package guide for more details.

Formatting units

xarray-units converts units to various string formats:

da = xr.DataArray([1, 2, 3]).units.set("m / s^2")

da_console = da.units.format("console")
da_latex = da.units.format("latex")

print(da_console)
print(da_latex)

<xarray.DataArray (dim_0: 3)>
array([1, 2, 3])
Dimensions without coordinates: dim_0
Attributes:
    units:    m s^-2

<xarray.DataArray (dim_0: 3)>
array([1, 2, 3])
Dimensions without coordinates: dim_0
Attributes:
    units:    $\mathrm{\frac{m}{s^{2}}}$

This is useful, for example, when plotting a DataArray:

da.units.format("latex").plot()

[!NOTE] By default, the units of the DataArray coordinates will also be formatted.

Advanced usages

Handling units of coordinates

The units accessor has an option for handling units of DataArray coordinates. For example, the following code will create a DataArray with x and y coordinates in units of meters:

da_m = xr.DataArray([[1, 2], [3, 4]], dims=["x", "y"]).units.set("deg_C")
da_m = da_m.assign_coords(
    x=xr.DataArray([1000, 2000], dims="x").units.set("m"),
    y=xr.DataArray([3000, 4000], dims="y").units.set("m"),
)
print(da_m.x)
print(da_m.y)

<xarray.DataArray 'x' (x: 2)>
array([1000, 2000])
Coordinates:
  * x        (x) int64 1000 2000
Attributes:
    units:    m

<xarray.DataArray 'y' (y: 2)>
array([3000, 4000])
Coordinates:
  * y        (y) int64 3000 4000
Attributes:
    units:    m

To handling the units of the DataArray coordinates, use an option of for specifying them:

da_km = da_m.units(of=["x", "y"]).to("km")
print(da_km.x)
print(da_km.y)

<xarray.DataArray 'x' (x: 2)>
array([1., 2.])
Coordinates:
  * x        (x) float64 1.0 2.0
Attributes:
    units:    km

<xarray.DataArray 'y' (y: 2)>
array([3., 4.])
Coordinates:
  * y        (y) float64 3.0 4.0
Attributes:
    units:    km

where of accepts the name(s) of the coordinate(s).

Method and operation chains

The units accessor has an option chain for chaining methods or operations while considering units:

da_m = xr.DataArray([1, 2, 3]).units.set("m")
da_s = xr.DataArray([1, 2, 3]).units.set("s")
da_a = da_m.units(chain=2) / da_s / da_s
print(da_a)

<xarray.DataArray (dim_0: 3)>
array([1.        , 0.5       , 0.33333333])
Dimensions without coordinates: dim_0
Attributes:
    units:    m / s2

where chain is the number of chained methods or operations. This is equivalent to nesting the units accessors:

(da_m.units / da_s).units / da_s

Use with static type checking

xarray-units provides a special type hint xarray_units.DataArray with which type checkers can statically handle the units accessor and its methods:

from xarray_units import DataArray

da: DataArray = xr.DataArray([1, 2, 3]).units.set("km")

[!TIP] xarray_units.DataArray will be replaced by xarray.DataArray at runtime, so it can also be used for creating and subclassing DataArray.

Use without the units accessor

xarray-units provides a function xarray_units.units that returns the units accessor of a DataArray. The following two codes are therefore equivalent:

xr.DataArray([1, 2, 3]).units.set("km")

from xarray_units import units

units(xr.DataArray([1, 2, 3])).set("km")