Add an example for pyproject.toml and move costly import

doc/source/developing/extensions.rst

@@ -40,16 +40,22 @@ 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:
+``yt.frontends`` namespace in ``setup.py``:
 
 .. code-block:: python
 
    setup(
-       entry_points={
-           "yt.frontends": ["myFrontend = my_frontend.api.MyFrontendDataset"]
-       }
+       # ...,
+       entry_points={"yt.frontends": ["myFrontend = my_frontend.api.MyFrontendDataset"]}
    )
 
+or ``pyproject.toml``:
+
+.. code-block:: toml
+
+   [project.entry-points."yt.frontends"]
+   myFrontend = "my_frontend.api.MyFrontendDataset"
+
 will be automatically loaded and immediately available in
 :func:`~yt.loaders.load`.
 

yt/loaders.py

@@ -12,7 +12,6 @@
 from multiprocessing import Pipe, Process
 from multiprocessing.connection import Connection
 from pathlib import Path
-from pkg_resources import iter_entry_points
 from typing import Dict, List, Optional, Tuple, Union
 from urllib.parse import urlsplit
 
@@ -98,6 +97,9 @@ def load(
     if not fn.startswith("http"):
         fn = str(lookup_on_disk_data(fn))
 
+    # import it here cause it's costly
+    from pkg_resources import iter_entry_points
+
     # Ensure that external frontends are loaded
     for entrypoint in iter_entry_points("yt.frontends"):
         entrypoint.load()

yt/tests/test_external_frontends.py

@@ -1,4 +1,4 @@
-import mock
+from unittest import mock
 
 from yt.data_objects.static_output import Dataset
 from yt.geometry.grid_geometry_handler import GridIndex