tests: add some ModifierButtons unit tests

tests/putil/test_modifierbuttons.py

@@ -0,0 +1,35 @@
+from panda3d.core import ModifierButtons
+
+
+def test_modifierbuttons_empty():
+    # Tests the initial state of a ModifierButtons object.
+    btns = ModifierButtons()
+    assert btns == ModifierButtons(btns)
+    assert btns != ModifierButtons()
+    assert btns.matches(ModifierButtons())
+    assert not btns.is_down("alt")
+    assert not btns.is_any_down()
+    assert not btns.has_button("alt")
+    assert btns.get_prefix() == ""
+    assert btns.get_num_buttons() == 0
+    assert len(btns.buttons) == 0
+
+
+def test_modifierbuttons_cow():
+    # Tests the copy-on-write mechanism of the button list.
+    btns1 = ModifierButtons()
+    btns1.add_button("space")
+
+    # Modifying original should not affect copy
+    btns2 = ModifierButtons(btns1)
+    assert tuple(btns2.buttons) == tuple(btns1.buttons)
+    btns1.add_button("enter")
+    assert tuple(btns1.buttons) == ("space", "enter")
+    assert tuple(btns2.buttons) == ("space",)
+
+    # Modifying copy should not affect original
+    btns3 = ModifierButtons(btns2)
+    assert tuple(btns3.buttons) == tuple(btns2.buttons)
+    btns3.add_button("escape")
+    assert tuple(btns2.buttons) == ("space",)
+    assert tuple(btns3.buttons) == ("space", "escape")