id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
2135011804,I_kwDOAMm_X85_QbHc,8748,release v2024.02.0,2448579,closed,0,14808389,,0,2024-02-14T19:08:38Z,2024-02-18T22:52:15Z,2024-02-18T22:52:15Z,MEMBER,,,,"### What is your issue?

Thanks to @keewis for volunteering at today's meeting :()","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/8748/reactions"", ""total_count"": 3, ""+1"": 0, ""-1"": 0, ""laugh"": 1, ""hooray"": 0, ""confused"": 0, ""heart"": 2, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
956259734,MDU6SXNzdWU5NTYyNTk3MzQ=,5649,xr.merge bug? when using combine_attrs='drop_conflicts',14314623,open,0,14808389,,3,2021-07-29T22:47:43Z,2022-04-09T03:14:24Z,,CONTRIBUTOR,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**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?

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


</details>
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