Add tests

Author: basnijholt

tests/test_widget.py

@@ -0,0 +1,7 @@
+from graphviz_anywidget import GraphvizWidget
+
+
+def test_widget() -> None:
+    dot_string = "digraph { a -> b; b -> c; c -> a; }"
+    pipe_func_graph_widget = GraphvizWidget(dot_source=dot_string)
+    assert pipe_func_graph_widget.dot_source == dot_string