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/2512#issuecomment-433194914,https://api.github.com/repos/pydata/xarray/issues/2512,433194914,MDEyOklzc3VlQ29tbWVudDQzMzE5NDkxNA==,6628425,2018-10-25T20:30:49Z,2018-10-25T20:30:49Z,MEMBER,"I think this is a bug.  The error message in particular makes things confusing.  The problem stems from this line, which results in an array of dtype object (see the explanation below):
```python
dts = np.array([dt + timedelta(days=x) for x in range(10)])
```
As a temporary workaround, if you keep `dts` as a list before passing it to the Dataset constructor, things will work as you expect:
```python
dts = [dt + timedelta(days=x) for x in range(10)]
```
----

By default NumPy will cast a list of `datetime.datetime` objects to an array with dtype object:
```
In [4]: dts = np.array([dt + timedelta(days=x) for x in range(10)])

In [5]: dts
Out[5]:
array([datetime.datetime(1999, 1, 1, 0, 0),
       datetime.datetime(1999, 1, 2, 0, 0),
       datetime.datetime(1999, 1, 3, 0, 0),
       datetime.datetime(1999, 1, 4, 0, 0),
       datetime.datetime(1999, 1, 5, 0, 0),
       datetime.datetime(1999, 1, 6, 0, 0),
       datetime.datetime(1999, 1, 7, 0, 0),
       datetime.datetime(1999, 1, 8, 0, 0),
       datetime.datetime(1999, 1, 9, 0, 0),
       datetime.datetime(1999, 1, 10, 0, 0)], dtype=object)
```
If you specify an array of dtype object as a coordinate, xarray currently has some logic that requires that it be cast to a generic `pandas.Index` with dtype object (and will preserve the `datetime.datetime` elements of the array).  
```
In [14]: da = xarray.DataArray(range(10), coords=[dts], dims=['time'])

In [15]: da
Out[15]:
<xarray.DataArray (time: 10)>
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
Coordinates:
  * time     (time) object 1999-01-01 1999-01-02 1999-01-03 1999-01-04 ...

In [16]: da.indexes['time']
Out[16]:
Index([1999-01-01 00:00:00, 1999-01-02 00:00:00, 1999-01-03 00:00:00,
       1999-01-04 00:00:00, 1999-01-05 00:00:00, 1999-01-06 00:00:00,
       1999-01-07 00:00:00, 1999-01-08 00:00:00, 1999-01-09 00:00:00,
       1999-01-10 00:00:00],
      dtype='object', name='time')
```
The code where this happens is here: https://github.com/pydata/xarray/blob/b2a377f8dd215a0e1d3cbab61e6ae930347f4063/xarray/core/utils.py#L69-L73
In practice, in the event that an object type array contains `datetime.datetime` objects, this should probably convert them to `np.datetime64` instead and return a `pandas.DatetimeIndex`.  This would be more consistent with how xarray currently handles passing an object array of `datetime.datetime` objects as the data argument to a DataArray; there it converts things automatically:
```
In [18]: xarray.DataArray(dts)
Out[18]:
<xarray.DataArray (dim_0: 10)>
array(['1999-01-01T00:00:00.000000000', '1999-01-02T00:00:00.000000000',
       '1999-01-03T00:00:00.000000000', '1999-01-04T00:00:00.000000000',
       '1999-01-05T00:00:00.000000000', '1999-01-06T00:00:00.000000000',
       '1999-01-07T00:00:00.000000000', '1999-01-08T00:00:00.000000000',
       '1999-01-09T00:00:00.000000000', '1999-01-10T00:00:00.000000000'],
      dtype='datetime64[ns]')
Dimensions without coordinates: dim_0
```
Xarray's logic to encode dates (i.e. what is used when saving datetime data to files) requires that dates are either of type `np.datetime64` or `cftime.datetime` (`datetime.datetime` objects are not supported there).  If `datetime.datetime` arrays were automatically converted to `np.datetime64` arrays in all cases then this would not be an issue.

The error message actually results from the dates being converted to `np.datetime64` *after* the datetime encoding logic is skipped (it happens in the `Variable` constructor [here](https://github.com/pydata/xarray/blob/master/xarray/coding/times.py#L397)) and netCDF4 doesn't know how to deal with that data type, but I think the fundamental issue relates to the lines of code I referenced above.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,374070147