Proxy Design Pattern
Videos
| Section | Video Links |
|---|---|
| Proxy Overview |    |
| Proxy Use Case | |
| __class__ Attribute | |
| Circular Imports | |
Book
Overview
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Terminology
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Proxy UML Diagram
Output
python ./proxy/proxy_concept.py 1848118706080 pulling data from RealSubject [1, 2, 3] pulling data from Proxy cache [1, 2, 3]
Example Use Case
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Example UML Diagram
Output
python ./proxy/client.py I am the form of a Lion I am the form of a Leopard I am the form of a Serpent I am the form of a Leopard I am the form of a Lion
New Coding Concepts
Changing An Objects Class At Runtime.
You change the class of an object by running self.__class__ = SomeOtherClass
Note that doing this does not affect any variables created during initialisation, eg self.variable_name = 'abc', since the object itself hasn't changed. Only its class methods and static attributes have been replaced with the class methods and static attributes of the other class.
This explains how calling tell_me_the_future() and tell_me_your_form() produced different results after changing self.__class__
Avoiding Circular Imports.
Normally in all the examples so far, I have been importing using the form
In /proxy/client.py I import the Lion module. The Lion module itself imports the Leopard and Serpent modules, which in turn also re import the Lion module again. This is a circular import and occurs in some situations when you separate your modules into individual files.
Circular imports will prevent the python interpreter from compiling your .py file into byte code.
The error will appear like,
cannot import name 'Lion' from partially initialized module 'lion' (most likely due to a circular import)
To avoid circular import errors, you can import modules using the form.
and when the import is actually needed in some method
OBJECT = module.ClassName
See the Lion, Serpent and Leopard classes for examples.
Summary
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