SubclassShadowing.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
When an object has an attribute that shares its name with a method on the object's class (or another class attribute), the instance attribute is 
prioritized during attribute lookup, shadowing the method.  

If a method on a subclass is shadowed by an attribute on a superclass in this way, this may lead to unexpected results or errors, as this
shadowing behavior is nonlocal and may be unintended.
</p>

</overview>
<recommendation>

<p>
Ensure method names on subclasses don't conflict with attribute names on superclasses, and rename one.  
If the shadowing behavior is intended, ensure this is explicit in the superclass.
</p>

</recommendation>
<example>
<p>
In the following example, the <code>_foo</code> attribute of class <code>A</code> shadows the method <code>_foo</code> of class <code>B</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/SubclassShadowingBad.py" />

<p>
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>