id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
771382653,MDU6SXNzdWU3NzEzODI2NTM=,4714,Allow sel's method and tolerance to vary per-dimension,18488,open,0,,,6,2020-12-19T13:37:36Z,2020-12-19T16:58:20Z,,NONE,,,,"Imagine some data like this:

```python
sensor_data = xr.DataArray(np.arange(6).reshape((3, 2)), coords=[
    ('time', [0, 2, 3]),
    ('sensor', ['A', 'C']),
])
```

Let's say we now want to sample these sensors at some arbitrary points. We can use vectorized indexing do this:
```python
sensor_data.sel({
    'sensor': xr.DataArray(['A', 'A', 'A', 'B', 'C'], dims=['sample']),
    'time': xr.DataArray([0, 1, 2, 0, 0], dims=['sample'])
})
```

This fails because we are sampling one of our sensors at time 1, where we don't have any observations. We can add `method='ffill'` to fix this:
```python
sensor_data.sel({
    'sensor': xr.DataArray(['A', 'A', 'A', 'B', 'C'], dims=['sample']),
    'time': xr.DataArray([0, 1, 2, 0, 0], dims=['sample'])
}, method='ffill')
# array([0, 0, 2, 0, 1])
```

The problem is that the bogus sensor ""B"" is now getting a value ffilled from sensor ""A""'s time 0 observation, which doesn't make a lot of sense because sensor names are arbitary. What we really want to do is apply the ffill only down the ""time"" array, so the `sel` call sitll fails if a sensor name is unknown but we can still benefit from ffilling in places where it makes sense.

So, it would be nice if we could supply a per-dimension `method` (or `tolerance`) like this:

```python
sensor_data.sel({
    'sensor': xr.DataArray(['A', 'A', 'A', 'B', 'C'], dims=['sample']),
    'time': xr.DataArray([0, 1, 2, 0, 0], dims=['sample'])
}, method={'time': 'ffill'})
```

From an implementation point of view, this looks like an easy addition in `indexing.remap_label_indexers`: https://github.com/pydata/xarray/blob/235b2e5bcec253ca6a85762323121d28c3b06038/xarray/core/indexing.py#L243","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4714/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue