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
932393011,MDExOlB1bGxSZXF1ZXN0Njc5NzQyODk1,5548,Fix 2 type annotations in dataarray.py,28426758,closed,0,,,2,2021-06-29T09:03:23Z,2021-06-29T19:15:33Z,2021-06-29T18:51:19Z,CONTRIBUTOR,,0,pydata/xarray/pulls/5548,"Both `set_index` and `reset_index` are wrappers to other methods that return `""DataArray""`, not `Optional[""DataArray""]`. That is, they will never return None. That's why these methods should also have only `""DataArray""` in there return signature. This way it will be possible to do something like `myarray = myarray.reset_index(...)` without getting a complaint from Mypy.

For extended discussion, see https://github.com/pydata/xarray/issues/5533#issuecomment-869822366

<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Closes #5533
- [ ] Tests added
- [ ] Passes `pre-commit run --all-files`
- [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
- [ ] New functions/methods are listed in `api.rst`
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930191953,MDU6SXNzdWU5MzAxOTE5NTM=,5533,Can `reset_index` return None?,28426758,closed,0,,,4,2021-06-25T13:48:04Z,2021-06-29T18:51:19Z,2021-06-29T18:51:19Z,CONTRIBUTOR,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
-->

**What happened**:

The line
```
img = img.reset_index([""spatial""])
```
was not accepted by mypy. Mypy gave the following error:
```
redacted.py:69: error: Incompatible types in assignment (expression has type ""Optional[DataArray]"", variable has type ""DataArray"")
```
**What you expected to happen**:

I think that the return type of `DataArray.reset_index` should be `DataArray`, not `Optional[DataArray]`. `Optional[Array]` also includes `None` but the source code doesn't return None anywhere as far as I can see. Specifically, `reset_index` just redirects to `_replace` which returns a `DataArray` not `Optional[DataArray]` according to its type annotation.

**Relevant code in `xarray.core.dataarray.py`**:

```python
class DataArray(AbstractArray, DataWithCoords, DataArrayArithmetic):
.
.
.
    def _replace(
        self,
        variable: Variable = None,
        coords=None,
        name: Union[Hashable, None, Default] = _default,
        indexes=None,
    ) -> ""DataArray"":
        if variable is None:
            variable = self.variable
        if coords is None:
            coords = self._coords
        if name is _default:
            name = self.name
        return type(self)(variable, coords, name=name, fastpath=True, indexes=indexes)
.
.
.
    def reset_index(
        self,
        dims_or_levels: Union[Hashable, Sequence[Hashable]],
        drop: bool = False,
    ) -> Optional[""DataArray""]:
        """"""Reset the specified index(es) or multi-index level(s).
        Parameters
        ----------
        dims_or_levels : hashable or sequence of hashable
            Name(s) of the dimension(s) and/or multi-index level(s) that will
            be reset.
        drop : bool, optional
            If True, remove the specified indexes and/or multi-index levels
            instead of extracting them as new coordinates (default: False).
        Returns
        -------
        obj : DataArray
            Another dataarray, with this dataarray's data but replaced
            coordinates.
        See Also
        --------
        DataArray.set_index
        """"""
        coords, _ = split_indexes(
            dims_or_levels, self._coords, set(), self._level_coords, drop=drop
        )
        return self._replace(coords=coords)
```

**Question**:
Do I need to worry that there is any input for `img`, where `img.reset_index` would return `None`?

If I understand everything correctly and this is just a bug, I'm happy to put in a pull request to change the annotation.

**Environment**:

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

<!-- Paste the output here xr.show_versions() here -->
INSTALLED VERSIONS
------------------
commit: None
python: 3.9.4 (default, Apr  9 2021, 16:34:09) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 5.4.0-73-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.0
libnetcdf: 4.7.4

xarray: 0.18.2
pandas: 1.2.4
numpy: 1.19.5
scipy: 1.6.1
netCDF4: 1.5.6
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.5.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.2.4
cfgrib: None
iris: None
bottleneck: None
dask: installed
distributed: None
matplotlib: 3.4.2
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 52.0.0.post20210125
pip: 21.0.1
conda: None
pytest: 6.2.4
IPython: 7.24.1
sphinx: 4.0.2

</details>
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