Interpolation always returns floats

[Illviljan]

What happened: When interpolating datasets integer arrays are forced to floats.

What you expected to happen: To retain the same dtype after interpolation.

Minimal Complete Verifiable Example:

import numpy as np
import dask.array as da
a = np.arange(0, 2)
b = np.core.defchararray.add("long_variable_name", a.astype(str))
coords = dict(time=da.array([0, 1]))
data_vars = dict()
for v in b:
    data_vars[v] = xr.DataArray(
        name=v,
        data=da.array([0, 1], dtype=int),
        dims=["time"],
        coords=coords,
    )
ds1 = xr.Dataset(data_vars)

print(ds1)
Out[35]: 
<xarray.Dataset>
Dimensions:              (time: 4)
Coordinates:
  * time                 (time) float64 0.0 0.5 1.0 2.0
Data variables:
    long_variable_name0  (time) int32 dask.array<chunksize=(4,), meta=np.ndarray>
    long_variable_name1  (time) int32 dask.array<chunksize=(4,), meta=np.ndarray>

# Interpolate:
ds1 = ds1.interp(
    time=da.array([0, 0.5, 1, 2]),
    assume_sorted=True,
    method="linear",
    kwargs=dict(fill_value="extrapolate"),
)

# dask array thinks it's an integer array:
print(ds1.long_variable_name0)
Out[55]: 
<xarray.DataArray 'long_variable_name0' (time: 4)>
dask.array<dask_aware_interpnd, shape=(4,), dtype=int32, chunksize=(4,), chunktype=numpy.ndarray>
Coordinates:
  * time     (time) float64 0.0 0.5 1.0 2.0

#  But once computed it turns out is a float:
print(ds1.long_variable_name0.compute())
Out[38]: 
<xarray.DataArray 'long_variable_name0' (time: 4)>
array([0. , 0.5, 1. , 2. ])
Coordinates:
  * time     (time) float64 0.0 0.5 1.0 2.0

Anything else we need to know?: An easy first step is to also force np.float_ in da.blockwise in missing.interp_func.

The more difficult way is to somehow be able to change back the dataarrays into the old dtype without affecting performance. I did a test simply adding .astype()to the returned value in missing.interp and it doubled the calculation time.

I was thinking the conversion to floats in scipy could be avoided altogether by adding a (non-)public option to ignore any dtype checks and just let the user handle the "unsafe" interpolations.

Related: https://github.com/scipy/scipy/issues/11093

Environment:

Output of xr.show_versions()

xr.show_versions() INSTALLED VERSIONS ------------------ commit: None python: 3.8.5 (default, Sep 3 2020, 21:29:08) [MSC v.1916 64 bit (AMD64)] python-bits: 64 OS: Windows libhdf5: 1.10.4 libnetcdf: None xarray: 0.16.2 pandas: 1.1.5 numpy: 1.17.5 scipy: 1.4.1 netCDF4: None pydap: None h5netcdf: None h5py: 2.10.0 Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: 1.3.2 dask: 2020.12.0 distributed: 2020.12.0 matplotlib: 3.3.2 cartopy: None seaborn: 0.11.1 numbagg: None pint: None setuptools: 51.0.0.post20201207 pip: 20.3.3 conda: 4.9.2 pytest: 6.2.1 IPython: 7.19.0 sphinx: 3.4.0

[mathause]

4771 forces the dtype to np.float_ for consistency. Leaving this open for the bigger issue: keeping the dtype (if possible).