Add tests

core/src/toga/__init__.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import importlib
-import importlib.util
 import warnings
 from pathlib import Path
 

core/tests/test_import.py

@@ -0,0 +1,38 @@
+import sys
+
+import pytest
+
+
+def test_lazy_succeed(monkeypatch):
+    """Submodules are imported on demand."""
+    for mod_name in ["toga", "toga.documents", "toga.widgets.button"]:
+        monkeypatch.delitem(sys.modules, mod_name, raising=False)
+
+    # A clean import of the top-level toga module should not import any submodules.
+    import toga
+
+    assert "toga.documents" not in sys.modules
+    assert "toga.widgets.button" not in sys.modules
+
+    # Accessing a name should import only the necessary submodules.
+    Button = toga.Button
+    assert "toga.widgets.button" in sys.modules
+    assert "toga.documents" not in sys.modules
+
+    # Accessing a name multiple times should return the same object.
+    assert Button is toga.Button
+    assert Button is sys.modules["toga.widgets.button"].Button
+
+    # Same again with a different module.
+    Document = toga.Document
+    assert Document is sys.modules["toga.documents"].Document
+
+
+def test_lazy_fail():
+    """Nonexistent names should raise a normal AttributeError."""
+    import toga
+
+    with pytest.raises(
+        AttributeError, match="module 'toga' has no attribute 'nonexistent'"
+    ):
+        toga.nonexistent