issues

What happened: When using map_blocks with a function which has non-xarray arguments before arguments that are xarray dataarray (e.g. arg1 is a xarray object, arg2 is not xarray, and arg3 is a xarray dataarray), the code fails to convert the dataarray argument to dataset and triggers downstream failure. The downstream failure occurs because ds.chunks returns a dict, whereas da.chunks returns a tuple.

What you expected to happen: The code intends to convert dataarrays to datasets before calling .chunks, and I expect it to do so.

Minimal Complete Verifiable Example:

``` import xarray as xr import pandas as pd import numpy as np import string

def random_point_data(n_points=1, n_times=100): size = (n_times, n_points) dims = ('time', 'point') times = pd.date_range('1979-01-01', freq='1D', periods=n_times) da = xr.DataArray(np.random.random(size=size), dims=(dims), coords={'time': times}) return da

def mock_function(da1, non_xarray_input, da2): return da1

X = random_point_data(n_points=3).chunk({'point': 1}) out = xr.map_blocks(mock_function, X, args=['random_string', X]) ```

gives an error of

```

ValueError Traceback (most recent call last) <ipython-input-65-dea560baad18> in <module> 14 15 X = random_point_data(n_points=3).chunk({'point': 1}) ---> 16 out = xr.map_blocks(mock_function, X, args=['random_string', X])

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/parallel.py in map_blocks(func, obj, args, kwargs, template) 363 for arg in xarray_objs[1:]: 364 assert_chunks_compatible(npargs[0], arg) --> 365 input_chunks.update(arg.chunks) 366 input_indexes.update(arg.indexes) 367

ValueError: dictionary update sequence element #0 has length 1; 2 is required ```

Anything else we need to know?: This should be fixed with a one line change here

from xarray_objs = tuple( dataarray_to_dataset(arg) if is_da else arg for is_da, arg in zip(is_array, aligned) ) to xarray_objs = tuple( dataarray_to_dataset(arg) if isinstance(arg, xr.DataArray) else arg for arg in aligned )

This is because is_array is determined on all args regardless of whether the arg is a xarray object, and aligned has already been filtered down to xarray objects only.

Environment: