html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/4495#issuecomment-706766842,https://api.github.com/repos/pydata/xarray/issues/4495,706766842,MDEyOklzc3VlQ29tbWVudDcwNjc2Njg0Mg==,10194086,2020-10-11T20:54:41Z,2020-10-11T20:54:41Z,MEMBER,"Following the data through `combine_by_coords` to `_infer_concat_order_from_coords` the following happens 

https://github.com/pydata/xarray/blob/080caf4246fe2f4d6aa0c5dcb65a99b376fa669b/xarray/core/combine.py#L49


```python
import pandas as pd

indexes = [da1.indexes[""time""], da2.indexes[""time""]]

first_items = pd.Index([index[0] for index in indexes])
series = first_items.to_series()
series.rank(method=""dense"", ascending=ascending)
```
which returns
```python
Series([], dtype: float64)
```
thus it tries to iterate through an empty list. We probably need to set
```python
series.rank(method=""dense"", ascending=True, numeric_only=False)
```
which raises
```python
<__array_function__ internals> in lexsort(*args, **kwargs)

cftime/_cftime.pyx in cftime._cftime.datetime.__richcmp__()

TypeError: cannot compare cftime.DatetimeGregorian(2000, 1, 1, 0, 0, 0, 0) and cftime.DatetimeProlepticGregorian(2001, 1, 1, 0, 0, 0, 0) (different calendars)
```
but I wonder what test failures this brings...




","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,717217552
https://github.com/pydata/xarray/issues/4495#issuecomment-706749409,https://api.github.com/repos/pydata/xarray/issues/4495,706749409,MDEyOklzc3VlQ29tbWVudDcwNjc0OTQwOQ==,10194086,2020-10-11T18:43:48Z,2020-10-11T18:43:48Z,MEMBER,"To reproduce:

```python
import numpy as np
import xarray as xr

time1 = xr.cftime_range(""2000"", ""2000-12-31"", calendar=""standard"")
time2 = xr.cftime_range(""2001"", ""2001-12-31"", calendar=""proleptic_gregorian"")

da1 = xr.DataArray(np.random.randn(366), dims=""time"", coords=dict(time=time1)).to_dataset(name=""name"")
da2 = xr.DataArray(np.random.randn(365), dims=""time"", coords=dict(time=time2)).to_dataset(name=""name"")

xr.combine_by_coords([da1, da2])
```
returns
```python
...
StopIteration: 
```

Funny enough this does trigger the correct error:
```python
time3 = xr.cftime_range(""2000"", ""2000-12-31"", calendar=""proleptic_gregorian"")
da3 = xr.DataArray(np.random.randn(366), dims=""time"", coords=dict(time=time3)).to_dataset(name=""name"")

xr.combine_by_coords([da1, da3])
```

```python
...
TypeError: cannot compare cftime.DatetimeProlepticGregorian(2000, 1, 1, 0, 0, 0, 0) and cftime.DatetimeGregorian(2000, 1, 1, 0, 0, 0, 0) (different calendars)
```










","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,717217552
https://github.com/pydata/xarray/issues/4495#issuecomment-705552768,https://api.github.com/repos/pydata/xarray/issues/4495,705552768,MDEyOklzc3VlQ29tbWVudDcwNTU1Mjc2OA==,2448579,2020-10-08T13:02:59Z,2020-10-08T13:02:59Z,MEMBER,Either a wget script or code to create dummy files would be useful,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,717217552