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Delegation-Based Classes in 3.0: __getattr__ and built-ins

We met this issue briefly in our class tutorial in Chapter 27 and our delegation coverage in Chapter 30: classes that use the __getattr__ operator overloading method to delegate attribute fetches to wrapped objects will fail in Python 3.0 unless operator overloading methods are redefined in the wrapper class. In Python 3.0 (and 2.6, when new-style classes are used), the names of operator overloading methods implicitly fetched by built-in operations are not routed through generic attribute-interception methods. The __str__ method used by printing, for example, never invokes __getattr__. Instead, Python 3.0 looks up such names in classes and skips the normal runtime instance lookup mechanism entirely. To work around this, such methods must be redefined in wrapper classes, either by hand, with tools, or by definition in superclasses. We’ll revisit this gotcha in Chapters 37 and 38.