test: add unit tests for relative_import

tests/test_core/test_utils/test_importers.py

@@ -0,0 +1,40 @@
+import collections.abc
+
+import pytest
+
+from _plotly_utils.importers import relative_import
+
+
+def _make():
+    # Use the stdlib ``collections`` package as a stable lazy-import target.
+    return relative_import(
+        "collections",
+        rel_modules=[".abc"],
+        rel_classes=[".abc.Mapping"],
+    )
+
+
+def test_all_lists_module_and_class_leaf_names():
+    all_, _getattr, _dir = _make()
+    assert sorted(all_) == ["Mapping", "abc"]
+
+
+def test_getattr_lazily_imports_submodule():
+    _all, getattr_, _dir = _make()
+    assert getattr_("abc") is collections.abc
+
+
+def test_getattr_lazily_imports_class():
+    _all, getattr_, _dir = _make()
+    assert getattr_("Mapping") is collections.abc.Mapping
+
+
+def test_getattr_unknown_name_raises_attribute_error():
+    _all, getattr_, _dir = _make()
+    with pytest.raises(AttributeError):
+        getattr_("does_not_exist")
+
+
+def test_dir_returns_all():
+    all_, _getattr, dir_ = _make()
+    assert sorted(dir_()) == sorted(all_)