Initialize custom backends in open_dataset (#5684)

* Test using correct signature

* Initialize the class like in build_engine

* Update whats-new.rst

* Update whats-new.rst

* Add back self to classes

* Add some typing to the test, mypy should fail

* mypy failed as expected, should not now

Co-authored-by: keewis <keewis@users.noreply.github.com>

Author: Illviljan

doc/whats-new.rst

@@ -71,7 +71,10 @@ Bug fixes
 - Fixed performance bug where ``cftime`` import attempted within various core operations if ``cftime`` not
   installed (:pull:`5640`).
   By `Luke Sewell <https://github.com/lusewell>`_
-
+- When a custom engine was used in :py:func:`~xarray.open_dataset` the engine
+  wasn't initialized properly, causing missing argument errors or inconsistent
+  method signatures. (:pull:`5684`)
+  By `Jimmy Westling <https://github.com/illviljan>`_.
 - Numbers are properly formatted in a plot's title (:issue:`5788`, :pull:`5789`).
   By `Maxime Liquet <https://github.com/maximlt>`_.
 

xarray/backends/plugins.py

@@ -158,7 +158,7 @@ def get_backend(engine):
             )
         backend = engines[engine]
     elif isinstance(engine, type) and issubclass(engine, BackendEntrypoint):
-        backend = engine
+        backend = engine()
     else:
         raise TypeError(
             (

xarray/tests/test_backends_api.py

@@ -26,10 +26,11 @@ def test_custom_engine() -> None:
 
     class CustomBackend(xr.backends.BackendEntrypoint):
         def open_dataset(
+            self,
             filename_or_obj,
             drop_variables=None,
             **kwargs,
-        ):
+        ) -> xr.Dataset:
             return expected.copy(deep=True)
 
     actual = xr.open_dataset("fake_filename", engine=CustomBackend)