Estimated time

45 minutes

Level of difficulty

Medium

Objectives

• improving the student's skills in operating with metaclasses;
• improving the student's skills in operating with class variables and class methods.

Scenario

• Imagine you’ve been given a task to clean up the code of a system developed in Python – the code should be treated as legacy code;
• the system was created by a group of volunteers who worked with no clear “clean coding” rules;
• the system suffers from a problem: we don’t know in which order the classes are created, so it causes multiple dependency problems;
• your task is to prepare a metaclass that is responsible for:
• equipping all newly instantiated classes with time stamps, persisted in a class attribute named instantiation_time;
• equipping all newly instantiated classes with the get_instantiation_time() method. The method should return the value of the class attribute instantiation_time.

* The metaclass should have its own class variable (a list) that contains a list of the names of the classes instantiated by the metaclass (tip: append the class name in the __new__ method).

• Your metaclass should be used to create a few distinct legacy classes;
• create objects based on the classes;
• list the class names that are instantiated by your metaclass.