Class or Static Variables in Python
All objects share class or static variables. An instance or non-static variables are different for different objects (every object has a copy). For example, let a Computer Science Student be represented by a class CSStudent. The class may have a static variable whose value is “cse” for all objects. And class may also have non-static members like name and roll.
In C++ and Java, we can use static keywords to make a variable a class variable. The variables which don’t have a preceding static keyword are instance variables. See this for the Java example and this for the C++ example.
Features of Static Variables
- Static variables are allocated memory ones when the object for the class is created for the first time.
- Static variables are created outside of methods but inside a class
- Static variables can be accessed through a class but not directly with a instance.
- Static variables behavior doesn’t change for every object.
The Python approach is simple; it doesn’t require a static keyword.
Note: All variables which are assigned a value in the class declaration are class variables. And variables that are assigned values inside methods are instance variables.
Example:
Python
# Python program to show that the variables with a value # assigned in class declaration, are class variables # Class for Computer Science Studentclass CSStudent: stream = 'cse' # Class Variable def __init__(self,name,roll): self.name = name # Instance Variable self.roll = roll # Instance Variable # Objects of CSStudent classa = CSStudent('Geek', 1)b = CSStudent('Nerd', 2) print(a.stream) # prints "cse"print(b.stream) # prints "cse"print(a.name) # prints "Geek"print(b.name) # prints "Nerd"print(a.roll) # prints "1"print(b.roll) # prints "2" # Class variables can be accessed using class# name alsoprint(CSStudent.stream) # prints "cse" # Now if we change the stream for just a it won't be changed for ba.stream = 'ece'print(a.stream) # prints 'ece'print(b.stream) # prints 'cse' # To change the stream for all instances of the class we can change it # directly from the classCSStudent.stream = 'mech' print(a.stream) # prints 'ece'print(b.stream) # prints 'mech' |
Output:
cse cse Geek Nerd 1 2 cse ece cse ece mech
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