Make Item protocol runtime checkable

gaphas/item.py

@@ -1,6 +1,6 @@
 """Basic items."""
 from math import atan2
-from typing import Protocol, Sequence
+from typing import Protocol, Sequence, runtime_checkable
 
 from gaphas.canvas import Context
 from gaphas.connector import Handle, LinePort, Port
@@ -16,6 +16,7 @@ from gaphas.state import (
 )
 
 
+@runtime_checkable
 class Item(Protocol):
     @property
     def matrix(self) -> Matrix:

tests/test_item.py

@@ -4,7 +4,7 @@ The test check functionality of `Item.constraint` method, not
 constraints themselves.
 """
 
-from gaphas.item import Element, Line
+from gaphas.item import Element, Item, Line
 
 
 class Custom:
@@ -30,3 +30,15 @@ def test_can_pass_arbitrary_arguments_to_a_line(connections):
 
     assert t.custom == "custom"
     assert t._connections is connections
+
+
+def test_element_implements_item_protocol(connections):
+    element = Element(connections)
+
+    assert isinstance(element, Item)
+
+
+def test_line_implements_item_protocol(connections):
+    line = Line(connections)
+
+    assert isinstance(line, Item)