UPDATE: Please let me know whether this PR can be considered to be merged. If not I won't bother trying to fix failed tests such as:

would reformat /home/vsts/work/1/s/xarray/core/map.py would reformat /home/vsts/work/1/s/xarray/core/dataset.py would reformat /home/vsts/work/1/s/xarray/tests/test_map.py Oh no! 💥 💔 💥

Thanks

• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [x] New functions/methods are listed in api.rst

I implemented the top-level xarray.map function. It is a generalization of the Dataset.map method for those functions which take more than one DataArray as input. The function will be applied to the intersection of the variables in the datasets.

E.g: f = lambda a, b: b-a map([ds1, ds2], f) More details in the docstring.

(I probably messed up something with git as the commits listed below also include my earlier commits not related to this PR. But the list of files is clean, it only includes what I'd like to merge)

But it would be expected (at least by me) that it does.

Bug in conversion frompd.series

MCVE Code Sample

```python import pandas as pd import xarray as xr

data3 = pd.DataFrame([(1, 2, 1), (2, 2, 1)],columns=['x', 'B', 'A']) data3 = data3.set_index(['x']) data3.rename_axis('tag', axis=1, inplace=True) data3 = data3.stack() print(data3) print(xr.DataArray.from_series(data3).sel(tag='B')) ```

Expected Output

[2,2]

Problem Description

The same code gives the expected output in xarray 0.14.1 Also works correctly if "tag" dimension is properly sorted during DataFrame cretion.

Versions

MCVE Code Sample

```python import xarray as xr

da1 = xr.DataArray([1,2],dims=['x'],coords={'x':[0,1]})

print(da1.indexes) da1['x'].values = [1,2] print(da1.indexes) print('--------') print(da1['x'])

x: Int64Index([0, 1], dtype='int64', name='x') x: Int64Index([0, 1], dtype='int64', name='x') -------- <xarray.DataArray 'x' (x: 2)> array([1, 2]) Coordinates: * x (x) int32 1 2 ```

Expected Output

```python x: Int64Index([0, 1], dtype='int64', name='x') x: Int64Index([1, 2], dtype='int64', name='x')

<xarray.DataArray 'x' (x: 2)> array([1, 2]) Coordinates: * x (x) int32 1 2 ```

Problem Description

• .indexes gets out of sync with the values stored in the array
• this is unfortunate as alignment with other array is done through .indexes

The issue does not occur if I a, either do not call ".indexes" before setting the value, or b, call da1['x'] = [1,2] instead of da1['x'].values = [1,2]

Output of xr.show_versions()

Code Sample

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

def noop(da): return da

def expd(da): return da.expand_dims(['A'])

na=np.array([[1,2],[3,4]]) xa=xr.DataArray(na,dims=['A','B'])

This works:

r1 = xa.groupby('A').apply(noop) print("!!!!")

This doesn't:

r2 = xa.groupby('A').apply(expd) ```

Problem description

Is this intended behavior? The reason I would need this is to be able to increase the size of each group.

Expected Output

I expect r1 to be equivalent to r2

• [x] Closes #2835
• [X] Tests added
• [X] Fully documented, including whats-new.rst for all changes and api.rst for new API

This issue may be related to #1386

I hope this part of the stacktrace is sufficient to demonstrate my point:

File "C:\src\validation\main\risk_reporting\data_access\db_model.py", line 246, in get_data ds = ds.groupby('pvshort').apply(select_latest_pv) File "C:\Anaconda2\lib\site-packages\xarray\core\common.py", line 345, in groupby return self.groupby_cls(self, group, squeeze=squeeze) File "C:\Anaconda2\lib\site-packages\xarray\core\groupby.py", line 226, in init unique_values, group_indices = unique_value_groups(group, sort=sort) File "C:\Anaconda2\lib\site-packages\xarray\core\groupby.py", line 34, in unique_value_groups inverse, values = pd.factorize(ar, sort=sort) File "C:\Anaconda2\lib\site-packages\pandas\core\algorithms.py", line 552, in factorize values = _ensure_arraylike(values) File "C:\Anaconda2\lib\site-packages\pandas\core\algorithms.py", line 166, in _ensure_arraylike values = lib.list_to_object_array(values) TypeError: Argument 'obj' has incorrect type (expected list, got DataArray)