Automatically load external frontends in load()

doc/source/developing/extensions.rst

@@ -35,6 +35,32 @@ In subsequent versions, we plan to include in yt a catalog of known extensions
 and where to find them; this will put discoverability directly into the code
 base.
 
+Frontend as an extension
+------------------------
+
+Starting with version 4.2 of yt, any externally installed package that exports
+:class:`~yt.data_objects.static_output.Dataset` subclass as an entrypoint in
+``yt.frontends`` namespace in ``setup.py``:
+
+.. code-block:: python
+
+   setup(
+       # ...,
+       entry_points={"yt.frontends": ["myFrontend = my_frontend.api.MyFrontendDataset"]}
+   )
+
+or ``pyproject.toml``:
+
+.. code-block:: toml
+
+   [project.entry-points."yt.frontends"]
+   IdefixVtkDataset = "yt_idefix.data_structures:IdefixVtkDataset"
+   IdefixDmpDataset = "yt_idefix.data_structures:IdefixDmpDataset"
+   PlutoVtkDataset = "yt_idefix.data_structures:PlutoVtkDataset"
+
+will be automatically loaded and immediately available in
+:func:`~yt.loaders.load`.
+
 Extension Template
 ------------------
 

yt/loaders.py

@@ -97,6 +97,21 @@ def load(
     if not fn.startswith("http"):
         fn = str(lookup_on_disk_data(fn))
 
+    # import it here cause it's costly
+    from importlib.metadata import entry_points
+
+    # overly complicated dance to suppress spurious warning
+    # https://github.com/python/importlib_metadata/pull/278
+    entrypoints = entry_points()
+    if hasattr(entrypoints, "select"):
+        external_frontends = entrypoints.select(group="yt.frontends")
+    else:
+        external_frontends = entrypoints.get("yt.frontends", [])
+
+    # Ensure that external frontends are loaded
+    for entrypoint in external_frontends:
+        entrypoint.load()
+
     candidates = []
     for cls in output_type_registry.values():
         if cls._is_valid(fn, *args, **kwargs):

yt/tests/test_external_frontends.py

@@ -0,0 +1,42 @@
+import tempfile
+from unittest import mock
+
+from yt.data_objects.static_output import Dataset
+from yt.geometry.grid_geometry_handler import GridIndex
+from yt.loaders import load
+from yt.utilities.object_registries import output_type_registry
+
+
+class MockEntryPoint:
+    @classmethod
+    def load(cls):
+        class MockHierarchy(GridIndex):
+            grid = None
+
+        class MockDataset(Dataset):
+            _index_class = MockHierarchy
+
+            def _parse_parameter_file(self):
+                self.current_time = 1.0
+
+            def _set_code_unit_attributes(self):
+                self.length_unit = self.quan(1.0, "code_length")
+                self.mass_unit = self.quan(1.0, "code_mass")
+                self.time_unit = self.quan(1.0, "code_time")
+
+            @classmethod
+            def _is_valid(cls, filename, *args, **kwargs):
+                return True
+
+
+def test_external_frontend():
+    assert "MockDataset" not in output_type_registry
+
+    with tempfile.NamedTemporaryFile() as test_file:
+        with mock.patch("importlib.metadata.entry_points") as ep:
+            ep.return_value = {"yt.frontends": [MockEntryPoint]}
+            ds = load(test_file.name)
+            assert "MockDataset" in str(ds.__class__)
+
+    assert "MockDataset" in output_type_registry
+    output_type_registry.pop("MockDataset")