full_like, ones_like, zeros_like should retain subclasses

[gerritholl]

Code Sample,

# Your code here
import numpy
import xarray

class MyDataArray(xarray.DataArray):
    pass

da = MyDataArray(numpy.arange(5))
da2 = xarray.zeros_like(da)
print(type(da), type(da2))

Problem description

I would expect that type(da2) is type(da), but this is not the case. The type of da is always <class 'xarray.core.dataarray.DataArray'>. Rather, the output of this script is:

<class '__main__.MyDataArray'> <class 'xarray.core.dataarray.DataArray'>

Expected Output

I would hope as an output:

<class '__main__.MyDataArray'> <class '__main__.MyDataArray'>

In principle changing this could break people's code, so if a change is implemented it should probably be through an optional keyword argument to the full_like/ones_like/zeros_like family.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.0.final.0 python-bits: 64 OS: Linux OS-release: 2.6.32-754.el6.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8

xarray: 0.10.7
pandas: 0.23.2
numpy: 1.15.2
scipy: 1.1.0
netCDF4: 1.4.0
h5netcdf: 0.6.1
h5py: 2.8.0
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.18.1
distributed: 1.22.0
matplotlib: 3.0.0
cartopy: 0.16.0
seaborn: 0.9.0
setuptools: 39.2.0
pip: 18.0
conda: None
pytest: 3.2.2
IPython: 6.4.0
sphinx: None

[shoyer]

Are there other functions where we don't do this?

I would be happy to accept a fix here. I don't think we need a few keyword argument -- I expect most subclass users would consider this a bug rather than a feature.

We might consider adding an official subclassing APIs like pandas:
https://pandas.pydata.org/pandas-docs/stable/extending.html#extending-subclassing-pandas

[stale]

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[TomNicholas]

See #3980 - this issue should be solved as part of #3980, but see that issue for more general discussion of subclassing.

[max-sixty]

Shall we fold this into #3980? (i.e. I don't think we need an issue for enabling subclassing for each method :) )