Add extra example

Author: joefarebrother

python/ql/src/Classes/SubclassShadowing/SubclassShadowing.qhelp

@@ -26,12 +26,14 @@ In the following example, the <code>_foo</code> attribute of class <code>A</code
 Calls to <code>B()._foo()</code> will result in a <code>TypeError</code>, as <code>3</code> will be called instead. 
 </p>
 
-<sample src="examples/SubclassShadowingGood.py" />
+<sample src="examples/SubclassShadowingBad.py" />
 
 <p>
-In the following example...
+In the following example, the behavior of the <code>default</code> attribute being shadowed to allow for customization during initialization is
+intended in within the superclass <code>A</code>. Overriding <code>default</code> in the subclass <code>B</code> is then OK.
 </p>
 
+<sample src="examples/SubclassShadowingGood.py" />
 
 </example>
 </qhelp>

python/ql/src/Classes/SubclassShadowing/examples/SubclassShadowingBad.py

@@ -0,0 +1,9 @@
+class A:
+    def __init__(self):
+        self._foo = 3
+
+class B:
+    # BAD: _foo is shadowed by attribute A._foo
+    def _foo(self):
+        return 2
+

python/ql/src/Classes/SubclassShadowing/examples/SubclassShadowingGood.py

@@ -1,9 +1,15 @@
 class A:
-    def __init__(self):
-        self._foo = 3
-
-class B:
-    # BAD: _foo is shadowed by attribute A._foo
-    def _foo(self):
-        return 2
+    def __init__(self, default_func=None):
+        if default_func is not None:
+            self.default = default_func 
 
+    # GOOD: The shadowing behavior is explicitly intended in the superclass.
+    def default(self):
+        return []
+    
+class B(A):
+    
+    # Subclasses may override the method `default`, which will still be shadowed by the attribute `default` if it is set.
+    # As this is part of the expected behavior of the superclass, this is fine. 
+    def default(self):
+        return {}