Add NULL singleton tests

tests/structs/test_null.py

@@ -0,0 +1,33 @@
+"""Test behavior of the `types.NULL` singleton."""
+from __future__ import annotations
+
+from einspect import NULL
+from einspect.structs import PyTupleObject, PyTypeObject
+
+
+def test_func_ptr():
+    """Test that NULL can be used as a function pointer."""
+
+    t = PyTypeObject(int)
+    n = t.tp_as_number.contents
+    assert n.nb_add != NULL
+    # Null FuncPtr should equal NULL
+    assert n.nb_matrix_multiply == NULL
+    # We can set this to NULL, and it'll invoke type(FuncPtr)()
+    n.nb_matrix_multiply = NULL
+    assert n.nb_matrix_multiply == NULL
+
+
+def test_py_object():
+    """Test usage as a NULL PyObject pointer."""
+
+    s = PyTupleObject(
+        ob_refcnt=1,
+        ob_type=PyTypeObject(tuple).as_ref(),
+        ob_size=2,
+        ob_item=[NULL] * 2,
+    )
+    assert not s.ob_item[0]
+    assert not s.ob_item[1]
+    assert s.ob_item[0] == NULL
+    assert s.ob_item[1] == NULL