error with `Canvas().raster` on xarray 0.17

ALL software version info

The full list is available here, the gist is:

• Python 3.8
• xarray 0.17.0
• datashader 0.12.0

Description of expected behavior and the observed behavior

I expect Canvas().raster to return a DataArray object, instead it returns an error.

Complete, minimal, self-contained example code that reproduces the issue

Adapted from your documentation

from datashader import transfer_functions as tf, reductions as rd
import numpy as np, datashader as ds, xarray as xr

def f(x,y):
    return np.cos((x**2+y**2)**2)

def sample(fn, n=50, range_=(0.0,2.4)):
    xs = ys = np.linspace(range_[0], range_[1], n)
    x,y = np.meshgrid(xs, ys)
    z   = fn(x,y)
    return xr.DataArray(z, coords=[('y',ys), ('x',xs)])

da = sample(f)
ds.Canvas().raster(da, interpolate='linear')

Stack traceback and/or browser JavaScript console output

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-7e8b8b432665> in <module>
     12 
     13 da = sample(f)
---> 14 ds.Canvas().raster(da, interpolate='linear')

/opt/conda/lib/python3.8/site-packages/datashader/core.py in raster(self, source, layer, upsample_method, downsample_method, nan_value, agg, interpolate, chunksize, max_mem)
   1131         dims = [ydim, xdim]
   1132         attrs = dict(res=res[0])
-> 1133         if source._file_obj is not None and hasattr(source._file_obj, 'nodata'):
   1134             attrs['nodata'] = source._file_obj.nodata
   1135 

/opt/conda/lib/python3.8/site-packages/xarray/core/common.py in __getattr__(self, name)
    237                 with suppress(KeyError):
    238                     return source[name]
--> 239         raise AttributeError(
    240             "{!r} object has no attribute {!r}".format(type(self).__name__, name)
    241         )

AttributeError: 'DataArray' object has no attribute '_file_obj'

I think this is related to #4809.

[philippjfr]

This was always just a heuristic. I think we should support the major readers and conventions including attrs['NODATA'] and attrs['nodatavals'].

[ppwadhwa]

This should be fixed in 0.12.1, which is available on the pyviz channel, conda-forge, and pip.