What happened:

Indexing on DataArrays sometimes produce inconsistent behaviors on DataArrays indexed with a DatetimeIndex. There are cases (see code below) where copied versions of the array obtained with DataArray.copy() loose the index's freq attribute when calling isel(), whereas copied versions obtained using DataArray.sortby() do not.

What you expected to happen:

I would expect the freq not to be dropped from the axis when indexing the DataArray. Or at least, if it is dropped, it should always be dropped, and not depend on how the DataArray has been obtained (i.e., it should not depend on whether it's a copy() of another array).

Minimal Complete Verifiable Example:

```python import xarray as xr import pandas as pd import numpy as np from pandas.tseries.frequencies import to_offset

freq = to_offset('MS') time_index = pd.DatetimeIndex([pd.Timestamp('20130101') + i * freq for i in range(10)])

xa_orig = xr.DataArray(np.random.randn(10), dims=('time'), coords={'time': time_index})

xa_copy = xa_orig.copy()

xa_sorted = xa_orig.sortby('time')

Set freq on all DataArrays:

xa_orig.get_index('time').freq = freq xa_copy.get_index('time').freq = freq xa_sorted.get_index('time').freq = freq

Print to confirm the freq is set correctly:

print(xa_orig.get_index('time').freq) print(xa_copy.get_index('time').freq) print(xa_sorted.get_index('time').freq) ```

Output (OK - as expected) ```

<MonthBegin> <MonthBegin> <MonthBegin> ```

Now, try some indexing using isel() on slices: python print(xa_orig.isel({'time': slice(0, 5, None)}).get_index('time').freq) print(xa_copy.isel({'time': slice(0, 5, None)}).get_index('time').freq) # freq is dropped! print(xa_sorted.isel({'time': slice(0, 5, None)}).get_index('time').freq)

Output (Not as expected: freq is dropped inconsistently, only on the copied DataArray) ```

<MonthBegin> None <MonthBegin> ```

Now, try the same using isel() on lists: python print(xa_orig.isel({'time': [0, 1, 2, 3, 4]}).get_index('time').freq) print(xa_copy.isel({'time': [0, 1, 2, 3, 4]}).get_index('time').freq) print(xa_sorted.isel({'time': [0, 1, 2, 3, 4]}).get_index('time').freq)

Output (Not as expected: freq always dropped) ```

None None None ```

Anything else we need to know?:

Environment:

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

INSTALLED VERSIONS ------------------ commit: None python: 3.8.5 (default, Sep 4 2020, 02:22:02) [Clang 10.0.0 ] python-bits: 64 OS: Darwin OS-release: 20.5.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: None LOCALE: (None, 'UTF-8') libhdf5: None libnetcdf: None xarray: 0.19.0 pandas: 1.2.3 numpy: 1.19.5 scipy: 1.6.2 netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: 3.4.1 cartopy: None seaborn: None numbagg: None pint: None setuptools: 49.6.0.post20200814 pip: 21.1.3 conda: None pytest: None IPython: 7.22.0 sphinx: 3.2.1