Floating point comparisons in Python xarray
Numpy’s testing suite includes numpy.testing.assert_allclose(). assert_allclose() is powerful for comparing scalars and N-dimensional arrays. Xarray can most simply be thought of as a super-Numpy with metadata bringing powerful indexing and database-like methods, while retaining the speed of Numpy.
Continuous integration using Xarray is addressed with the
If you try to use
numpy.testing.assert_allclose on the frequently-used
xarray.Dataset, errors will result like:
TypeError: cannot directly convert an xarray.Dataset into a numpy array. Instead, create an xarray.DataArray first, either with indexing on the Dataset or by invoking the `to_array()` method.
The simple answer is to do something like:
import xarray import xarray.tests # run your simulation etc. dat = myfunc(...) # load the reference Dataset to compare against ref = xarray.open_dataset('ref.nc') xarray.tests.assert_allclose(ref, dat)
We didn’t use dat.equals(ref) Because it is generally inappropriate to directly compare floating point numbers, including the ubiquitous IEEE 754. Numerous authors have written on this subject for decades, and it’s an essential piece of knowledge for anyone doing computer engineering work. Clive Moler’s article address the topic of floating point comparisons succinctly.
floating point comparison algorithm
Across computing languages, an algorithm suitable for comparing floating point numbers actual, desired to absolute tolerance atol and relative tolerance rtol is:
isclose = abs(actual-desired) <= max(rtol * max(abs(actual), abs(desired)), atol)
isclose is Boolean True/False (an array if
desired are arrays.