Python OOP Tutorial 2: Class Variables

In this Python Object-Oriented Tutorial, we will be learning about class variables. We will see how they differ from instance variables and also some ideas for exactly how we would want to use them. Let's get started.
Python OOP 1 - Classes and Instances - • Python OOP Tutorial 1:...
Python OOP 2 - Class Variables - • Python OOP Tutorial 2:...
Python OOP 3 - Classmethods and Staticmethods - • Python OOP Tutorial 3:...
Python OOP 4 - Inheritance - • Python OOP Tutorial 4:...
Python OOP 5 - Special (Magic/Dunder) Methods - • Python OOP Tutorial 5:...
Python OOP 6 - Property Decorators - • Python OOP Tutorial 6:...
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Summary: In this video, Corey taught as how to differentiate between a Class variable and instance variable, how they are related to each, other, and when each of them is more useful over the other. Class variables are variables that we set inside a class, and are shared among all instances. Corey gave a good example where the number of total employs should be the same to every employ, no matter which employee we are referring to. Therefore, emp_1.num_of_employ = emp_2.num_of_employ = Employee.num_of_employ Instance variables are variables that are different from each instance. For example, the names and the pay for each employee. They have to be different. Corey also shows that class variables and instance variables are closely related, and that class variables are kind of 'inherited' to the 'self' variables. To illustrate this, Corey shows an example of 'annual raise of pay'. He initially creates the class variable to show a case where annual raise is equal among all the employees. This variable of 1.04 was accessible through each instance, and also through the class itself(obiviously). That is, print(Employee.annual_raise) print(emp_1.annual_raise) print(emp_2.anual_rais) all printed out 1.04. However, using the .__dict___ thing, Corey shows that the intances, emp_1 and emp_2 does not contain the annual_raise value. Corey explains that if a variable is not found within an instance and programmers try to access the variable, python automatically looks in in the variable of the instance's class, and then the more classes that the instance's class inherits from. Furthermore, if we access the class variable through an instance and then change it, python creates the variable within the instance. We can check it by using the .__dict__ thing. Corey shows that annual_raise key was created when he manually changed the annual_raise value as 1.05 in the following way. emp_1.annual_raise = 1.05 however, we know that the class variable remained the same at 1.04, when printing the class variable. print(Employee.annual_raise) ==> 1.04

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Thank you so much. I wanted to revise everything he said but did not want to watch the entire video again. Your comment helped me with just.

[@amansinghal2431]

in the second para you said that class variables are shared among all instances but I dont think thats the case. Corey said pyhton first searches the variables in the instance but if it doesnt find it, it goes to the class of the instance and returns it from their. if you dont explicitly mention emp1.raise_amount = 1.05, emp1 instance doesnt have the raise_amount vriables. Please confirm

[@infinitedeathloop5517]

@@amansinghal2431 I think he means, when the raise_amount variable isn't found in the instances, python searches for the instance in the class and applies it to both the instances, thus making them share the same variable. I hope I was clear

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Thankx a lot mate

[@aniruddhkhare2080]

@@amansinghal2431 The fact that class variables are shared among instances is correct. When class variables are used in methods using class names (as in Employee.raise_amount), then python directly accesses the class namespace, which is common for every instance. But when self is used (as in self.raise_amount), then python first searches the object's namespace, and if not found, then the class namespace. That's why Corey states that using self allows to assign different class variable values to different objects, as reassigning them (such as self.raise_amount = 1.05), makes the value a part of the object's namespace which is queried first.

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Thanks! Glad it helped!

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Thank you for this easy to follow tutorial, Corey!