ENH: automatically load external frontends in load()

doc/source/developing/extensions.rst

@@ -35,6 +35,37 @@ 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`` or ``pyproject.toml`` will be
+automatically loaded and immediately available in :func:`~yt.loaders.load`.
+
+To add an entrypoint in an external project's ``setup.py``:
+
+.. code-block:: python
+
+   setup(
+       # ...,
+       entry_points={
+           "yt.frontends": [
+               "myFrontend = my_frontend.api.MyFrontendDataset",
+               "myOtherFrontend = my_frontend.api.MyOtherFrontendDataset",
+           ]
+       }
+   )
+
+or ``pyproject.toml``:
+
+.. code-block:: toml
+
+   [project.entry-points."yt.frontends"]
+   myFrontend = "my_frontend.api:MyFrontendDataset"
+   myOtherFrontend = "my_frontend.api:MyOtherFrontendDataset"
+
+
 Extension Template
 ------------------
 

nose_unit.cfg

@@ -6,5 +6,5 @@ nologcapture=1
 verbosity=2
 where=yt
 with-timer=1
-ignore-files=(test_load_errors.py|test_load_sample.py|test_commons.py|test_ambiguous_fields.py|test_field_access_pytest.py|test_save.py|test_line_annotation_unit.py|test_eps_writer.py|test_registration.py|test_invalid_origin.py|test_outputs_pytest\.py|test_normal_plot_api\.py|test_load_archive\.py|test_stream_particles\.py|test_file_sanitizer\.py|test_version\.py|\test_on_demand_imports\.py|test_set_zlim\.py|test_add_field\.py|test_glue\.py|test_geometries\.py|test_firefly\.py|test_callable_grids\.py)
+ignore-files=(test_load_errors.py|test_load_sample.py|test_commons.py|test_ambiguous_fields.py|test_field_access_pytest.py|test_save.py|test_line_annotation_unit.py|test_eps_writer.py|test_registration.py|test_invalid_origin.py|test_outputs_pytest\.py|test_normal_plot_api\.py|test_load_archive\.py|test_stream_particles\.py|test_file_sanitizer\.py|test_version\.py|\test_on_demand_imports\.py|test_set_zlim\.py|test_add_field\.py|test_glue\.py|test_geometries\.py|test_firefly\.py|test_callable_grids\.py|test_external_frontends\.py)
 exclude-test=yt.frontends.gdf.tests.test_outputs.TestGDF

tests/tests.yaml

@@ -230,6 +230,7 @@ other_tests:
      - "--ignore-file=test_geometries\\.py"
      - "--ignore-file=test_firefly\\.py"
      - "--ignore-file=test_callable_grids\\.py"
+     - "--ignore-file=test_external_frontends\\.py"
      - "--exclude-test=yt.frontends.gdf.tests.test_outputs.TestGDF"
      - "--exclude-test=yt.frontends.adaptahop.tests.test_outputs"
      - "--exclude-test=yt.frontends.stream.tests.test_stream_particles.test_stream_non_cartesian_particles"

yt/frontends/sdf/data_structures.py

@@ -183,8 +183,11 @@ def _is_valid(cls, filename, *args, **kwargs):
             # Grab a whole 4k page.
             line = next(hreq.iter_content(4096))
         elif os.path.isfile(sdf_header):
-            with open(sdf_header, encoding="ISO-8859-1") as f:
-                line = f.read(10).strip()
+            try:
+                with open(sdf_header, encoding="ISO-8859-1") as f:
+                    line = f.read(10).strip()
+            except PermissionError:
+                return False
         else:
             return False
         return line.startswith("# SDF")

yt/loaders.py

@@ -9,6 +9,7 @@
 import time
 import types
 import warnings
+from importlib.metadata import entry_points
 from multiprocessing import Pipe, Process
 from multiprocessing.connection import Connection
 from pathlib import Path
@@ -97,6 +98,15 @@ def load(
     if not fn.startswith("http"):
         fn = str(lookup_on_disk_data(fn))
 
+    if sys.version_info >= (3, 10):
+        external_frontends = entry_points(group="yt.frontends")
+    else:
+        external_frontends = entry_points().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 +1,59 @@
+import sys
+
+import pytest
+
+import yt
+from yt.data_objects.static_output import Dataset
+from yt.geometry.grid_geometry_handler import GridIndex
+from yt.utilities.object_registries import output_type_registry
+
+
+class MockEntryPoint:
+    @classmethod
+    def load(cls):
+        class MockHierarchy(GridIndex):
+            grid = None
+
+        class ExtDataset(Dataset):
+            _index_class = MockHierarchy
+
+            def _parse_parameter_file(self):
+                self.current_time = 1.0
+                self.cosmological_simulation = 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 filename.endswith("mock")
+
+
+@pytest.fixture()
+def mock_external_frontend(monkeypatch):
+    def mock_entry_points(group=None):
+        if sys.version_info >= (3, 10):
+            return [MockEntryPoint]
+        else:
+            return {"yt.frontends": [MockEntryPoint]}
+
+    monkeypatch.setattr(yt.loaders, "entry_points", mock_entry_points)
+    assert "ExtDataset" not in output_type_registry
+
+    yield
+
+    assert "ExtDataset" in output_type_registry
+    # teardown to avoid test pollution
+    output_type_registry.pop("ExtDataset")
+
+
+@pytest.mark.usefixtures("mock_external_frontend")
+def test_external_frontend(tmp_path):
+    test_file = tmp_path / "tmp.mock"
+    test_file.write_text("")  # create the file
+    assert test_file.is_file()
+
+    ds = yt.load(test_file)
+    assert "ExtDataset" in ds.__class__.__name__