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Inner Class in Python

Inner Class in Python : Definition and Usage

In Python, an inner class is a class defined inside another class. Inner classes are used to logically group classes together. They are used to represent a relationship between the outer class and the inner class.

An inner class can access all the members of the outer class, but the reverse is not true. The outer class cannot access the members of the inner class.

Here is an example of an inner class in Python:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
outer = Outer()
inner = outer.Inner()
print(inner.inner_var)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
outer = Outer()
inner = outer.Inner()
print(inner.inner_var)

In the above example, we have defined an inner class InnerInner inside the outer class OuterOuter. The inner class InnerInner has its own variables and methods. We can create an instance of the inner class using the outer class instance.

When we run the above code, it will output:

Command
C:\Users\username\Desktop> python inner_class.py
Inner class variable
Command
C:\Users\username\Desktop> python inner_class.py
Inner class variable

Another way to create an instance of the inner class is by using the outer class name. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner().inner_method()
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner().inner_method()

Accessing Inner Class Members

You can access the members of the inner class using the object of the inner class. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)

In this example, we have created an object of the inner class InnerInner and accessed the inner_varinner_var variable using the object.

When we run the above code, it will output:

Command
C:\Users\username\Desktop> python inner_class.py
Inner class variable
Command
C:\Users\username\Desktop> python inner_class.py
Inner class variable

Types of Inner Classes

There are three types of inner classes in Python:

  • Regular Inner Class: A regular inner class is a class defined inside another class.
  • Static Inner Class: A static inner class is a class defined inside another class with the @staticmethod@staticmethod decorator.
  • Inner Class with Inheritance: An inner class can inherit from another class.

Regular Inner Class

A regular inner class is a class defined inside another class. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)

In the above example, we have defined a regular inner class InnerInner inside the outer class OuterOuter.

Static Inner Class

A static inner class is a class defined inside another class with the @staticmethod@staticmethod decorator. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    @staticmethod
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer.Inner()
print(inner.inner_var)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    @staticmethod
    class Inner:
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer.Inner()
print(inner.inner_var)

In the above example, we have defined a static inner class InnerInner inside the outer class OuterOuter using the @staticmethod@staticmethod decorator.

Inner Class with Inheritance

An inner class can inherit from another class. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner(Outer):
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner(Outer):
        def __init__(self):
            self.inner_var = "Inner class variable"
 
        def inner_method(self):
            print("Inner class method")
 
inner = Outer().Inner()
print(inner.inner_var)

In the above example, we have defined an inner class InnerInner that inherits from the outer class OuterOuter.

Types of Inner Classes (Categories by the level)

  • Multiple Inner Classes: A class can have multiple inner classes.
  • Multilevel Inner Classes: An inner class can have another inner class inside it.

Multiple Inner Classes

A class can have multiple inner classes. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner1:
        def __init__(self):
            self.inner_var1 = "Inner class variable 1"
 
        def inner_method1(self):
            print("Inner class method 1")
 
    class Inner2:
        def __init__(self):
            self.inner_var2 = "Inner class variable 2"
 
        def inner_method2(self):
            print("Inner class method 2")
 
inner1 = Outer().Inner1()
inner2 = Outer().Inner2()
print(inner1.inner_var1)
print(inner2.inner_var2)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner1:
        def __init__(self):
            self.inner_var1 = "Inner class variable 1"
 
        def inner_method1(self):
            print("Inner class method 1")
 
    class Inner2:
        def __init__(self):
            self.inner_var2 = "Inner class variable 2"
 
        def inner_method2(self):
            print("Inner class method 2")
 
inner1 = Outer().Inner1()
inner2 = Outer().Inner2()
print(inner1.inner_var1)
print(inner2.inner_var2)

In the above example, we have defined two inner classes Inner1Inner1 and Inner2Inner2 inside the outer class OuterOuter.

Multilevel Inner Classes

An inner class can have another inner class inside it. Here is an example:

inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner1:
        def __init__(self):
            self.inner_var1 = "Inner class variable 1"
 
        def inner_method1(self):
            print("Inner class method 1")
 
        class Inner2:
            def __init__(self):
                self.inner_var2 = "Inner class variable 2"
 
            def inner_method2(self):
                print("Inner class method 2")
 
inner1 = Outer().Inner1()
inner2 = inner1.Inner2()
print(inner2.inner_var2)
inner_class.py
class Outer:
    def __init__(self):
        self.outer_var = "Outer class variable"
 
    def outer_method(self):
        print("Outer class method")
 
    class Inner1:
        def __init__(self):
            self.inner_var1 = "Inner class variable 1"
 
        def inner_method1(self):
            print("Inner class method 1")
 
        class Inner2:
            def __init__(self):
                self.inner_var2 = "Inner class variable 2"
 
            def inner_method2(self):
                print("Inner class method 2")
 
inner1 = Outer().Inner1()
inner2 = inner1.Inner2()
print(inner2.inner_var2)

In the above example, we have defined an inner class Inner2Inner2 inside the inner class Inner1Inner1.

Conclusion

In this article, you learned about inner classes in Python. You also learned about the different types of inner classes and how to create and use them in Python. Inner classes are a powerful feature of Python that allows you to logically group classes together and represent relationships between classes. For more information, you can refer to the official documentation. For more tutorials, you can visit the Python Central Hub.

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