Do type checks on values we get from JS

qutebrowser/browser/webengine/webengineelem.py

@@ -39,6 +39,37 @@ class WebEngineElement(webelem.AbstractWebElement):
 
     def __init__(self, js_dict, tab):
         super().__init__(tab)
+        # Do some sanity checks on the data we get from JS
+        js_dict_types = {
+            'id': int,
+            'text': str,
+            'value': str,
+            'tag_name': str,
+            'outer_xml': str,
+            'class_name': str,
+            'rects': list,
+            'attributes': dict,
+        }
+        assert set(js_dict.keys()).issubset(js_dict_types.keys())
+        for name, typ in js_dict_types.items():
+            if name in js_dict and not isinstance(js_dict[name], typ):
+                raise TypeError("Got {} for {} from JS but expected {}: {}".format(
+                    type(js_dict[name]), name, typ, js_dict))
+        for name, value in js_dict['attributes'].items():
+            if not isinstance(name, str):
+                raise TypeError("Got {} ({}) for attribute name from JS: "
+                                "{}".format(name, type(name), js_dict))
+            if not isinstance(value, str):
+                raise TypeError("Got {} ({}) for attribute {} from JS: "
+                                "{}".format(value, type(value), name, js_dict))
+        for rect in js_dict['rects']:
+            assert set(rect.keys()) == {'top', 'right', 'bottom', 'left',
+                                        'height', 'width'}, rect.keys()
+            for value in rect.values():
+                if not isinstance(value, (int, float)):
+                    raise TypeError("Got {} ({}) for rect from JS: "
+                                    "{}".format(value, type(value), js_dict))
+
         self._id = js_dict['id']
         self._js_dict = js_dict