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
708766975,MDExOlB1bGxSZXF1ZXN0NDkyOTMxMzk3,4458,Autodetect the engine only when `engine=None`,226037,closed,0,,,2,2020-09-25T08:53:27Z,2020-12-10T10:30:56Z,2020-09-28T15:42:01Z,MEMBER,,0,pydata/xarray/pulls/4458,"Always use the `engine` explicitly selected by the user and perform auto detection only when `engine=None`.

 - [x] Closes #4457
 - [x] Tests added
 - [x] Passes `isort . && black . && mypy . && flake8`
 - [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
 - [x] No new functions/methods

The refactor complexity stems mostly from moving or duplicating some logic from `_get_engine_from_magic_number` to the backend stores. The current, rather ugly, code minimizes changes to the error conditions and messages.

Later in the backend refactor the plugins will provide the functionalities to identify the format from the file content and the duplication should be eliminated.

@jhamman do you think I need to acknowledge the CZI sponsorship in the changelog?","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4458/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
708459047,MDU6SXNzdWU3MDg0NTkwNDc=,4457,Silently ignoring user passed `engine` key in some cases when opening file objects,226037,closed,0,,,0,2020-09-24T20:37:37Z,2020-09-28T15:42:01Z,2020-09-28T15:42:01Z,MEMBER,,,,"**What happened**:

When I `open()` a netCDF4 file and pass the file object to `xr.open_dataset` with `engine='scipy'` xarray silently ignores the value of the `engine` keyword and opens the file with `h5netcdf`. When opening an netCDF3 file with `engine='h5netcdf'` the `scipy` engine is used instead without warning.

**What you expected to happen**:

Silent autodetection of the engine is fine when passing `engine=None`, but it is very surprising when passing a value explicitly. I would have expected an exception or at least a warning.

**Minimal Complete Verifiable Example**:

```python
>>> xr.open_dataset('netCDF4.nc', engine='scipy')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
[...]
TypeError: Error: netCDF4.nc is not a valid NetCDF 3 file
            If this is a NetCDF4 file, you may need to install the
            netcdf4 library, e.g.,

            $ pip install netcdf4

>>> fp = open('netCDF4.nc')
>>> xr.open_dataset(fp, engine='scipy')
<xarray.Dataset>
Dimensions:       (line: 41587, pixel: 17751)
Coordinates:
  * line          (line) int64 0 1 2 3 4 5 ... 41582 41583 41584 41585 41586
  * pixel         (pixel) int64 0 1 2 3 4 5 ... 17746 17747 17748 17749 17750
Data variables:
    phase         (line, pixel) float32 ...
Attributes:
    Conventions:             CF-1.7
    [...]
```
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