Friday, May 9, 2014

python template method; parent depending on child

Background -- classic template method pattern basically sets up a base class dependency on (by calling) a subclass method, provided the method is abstract in base class.

Example -- doHtml(), doParameters() and doProperties() methods are abstract in the base EMPanel class.

1) Python pushes the pattern further, when method can be completely _undeclared_ in base class.  See runCommand() in example on P222 [[Programming Python]].

* When you look at the base class in isolation, you don't know what self.runCommand() binds to. It turned out it's declared only in subclass.

2) Python pushes the pattern stillllll further, when _undeclared_ fields can be used in base class. The self.menu thing looks like a data field but undeclared. Well, it's declared in a subclass!

3) I have yet to try a simple example but multiple sources [3] say python pushes the pattern yeeeet further, when a method can be invoked without declaring it in any class -- if it's declared in an Instance. That instance effectively is an instance of an anonymous subclass (Java!).

* There's no compiler to please! At run time, python can "search" in instance and subclass scopes, using a turbo charged template-method search engine.

In conclusion, at creation time a python base class can freely reference any field or method even if base class doesn't include them in its member-listing.

[3] P96 [[ref]]

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