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:

Since recently, Darts uses DataArray as the core data structure underpinning its TimeSeries class to represent all time series in the library. It's working great for us, so I also want to take the chance to thank the xarray community here!