issues: 956259734

What happened: I have recently encountered a situation where combining two datasets failed, due to the datatype of their attributes. This example illustrates the situation: ```python ds1 = xr.Dataset(attrs={'a':[5]}) ds2 = xr.Dataset(attrs={'a':6})

xr.merge([ds1, ds2], combine_attrs='drop_conflicts') give me this error:

---

TypeError Traceback (most recent call last) <ipython-input-12-1c8e82be0882> in <module> 2 ds2 = xr.Dataset(attrs={'a':6}) 3 ----> 4 xr.merge([ds1, ds2], combine_attrs='drop_conflicts')

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/merge.py in merge(objects, compat, join, fill_value, combine_attrs) 898 dict_like_objects.append(obj) 899 --> 900 merge_result = merge_core( 901 dict_like_objects, 902 compat,

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/merge.py in merge_core(objects, compat, join, combine_attrs, priority_arg, explicit_coords, indexes, fill_value) 654 ) 655 --> 656 attrs = merge_attrs( 657 [var.attrs for var in coerced if isinstance(var, (Dataset, DataArray))], 658 combine_attrs,

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/merge.py in merge_attrs(variable_attrs, combine_attrs, context) 544 } 545 ) --> 546 result = { 547 key: value 548 for key, value in result.items()

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/merge.py in <dictcomp>(.0) 547 key: value 548 for key, value in result.items() --> 549 if key not in attrs or equivalent(attrs[key], value) 550 } 551 dropped_keys |= {key for key in attrs if key not in result}

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/utils.py in equivalent(first, second) 171 return duck_array_ops.array_equiv(first, second) 172 elif isinstance(first, list) or isinstance(second, list): --> 173 return list_equiv(first, second) 174 else: 175 return (

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/utils.py in list_equiv(first, second) 182 def list_equiv(first, second): 183 equiv = True --> 184 if len(first) != len(second): 185 return False 186 else:

TypeError: object of type 'int' has no len() ``` Took me a while to find out what the root cause of this was with a fully populated dataset, since the error is less than obvious.

What you expected to happen: In my understanding this should just drop the attribute a. The example works like expected when both attributes are an integer or both are lists with an integer. The error is only triggered when the type is mixed.

Is there a way to handle this case more elegantly?

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS ------------------ commit: None python: 3.8.8 | packaged by conda-forge | (default, Feb 20 2021, 16:22:27) [GCC 9.3.0] python-bits: 64 OS: Linux OS-release: 5.4.89+ machine: x86_64 processor: x86_64 byteorder: little LC_ALL: C.UTF-8 LANG: C.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.10.6 libnetcdf: 4.7.4 xarray: 0.19.1.dev8+gda99a566 pandas: 1.2.4 numpy: 1.20.2 scipy: 1.6.2 netCDF4: 1.5.6 pydap: installed h5netcdf: 0.11.0 h5py: 3.2.1 Nio: None zarr: 2.7.1 cftime: 1.4.1 nc_time_axis: 1.2.0 PseudoNetCDF: None rasterio: 1.2.2 cfgrib: 0.9.9.0 iris: None bottleneck: 1.3.2 dask: 2021.04.1 distributed: 2021.04.1 matplotlib: 3.4.1 cartopy: 0.19.0 seaborn: None numbagg: None pint: 0.17 setuptools: 49.6.0.post20210108 pip: 20.3.4 conda: None pytest: None IPython: 7.22.0 sphinx: None