Fix callback typo and add tests for it

src/watchpoints/watch.py

@@ -77,7 +77,7 @@ def tracefunc(self, frame, event, arg):
         for elem in self.watch_list:
             changed, exist = elem.changed(frame)
             if changed:
-                if elem.callback:
+                if elem._callback:
                     elem._callback(frame, elem, (self._prev_funcname, self._prev_filename, self._prev_lineno))
                 else:
                     self._callback(frame, elem, (self._prev_funcname, self._prev_filename, self._prev_lineno))

src/watchpoints/watch_element.py

@@ -25,7 +25,7 @@ def __init__(self, frame, node, alias=None, default_alias=None, printer=None, ca
         self.alias = alias
         self.default_alias = default_alias
         self.printer = printer
-        self.callback = callback
+        self._callback = callback
         self.exist = True
 
     def changed(self, frame):

tests/test_watch.py

@@ -75,6 +75,25 @@ def __init__(self):
         obj.a = 2
         self.assertEqual(cb.counter, 1)
 
+    def test_element_callback(self):
+        cb = CB()
+        a = [1, 2, 3]
+        watch(a, callback=cb)
+        a[0] = 2
+        a.append(4)
+        b = a
+        b.append(5)
+        a = {"a": 1}
+        a["b"] = 2
+
+        def change(d):
+            d["c"] = 3
+
+        change(a)
+
+        self.assertEqual(cb.counter, 6)
+        unwatch()
+
     def test_printer(self):
         watch.restore()
         s = io.StringIO()