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Bound methods python

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  • Difficulty Level : Expert
  • Last Updated : 11 Dec, 2019
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A bound method is the one which is dependent on the instance of the class as the first argument. It passes the instance as the first argument which is used to access the variables and functions. In Python 3 and newer versions of python, all functions in the class are by default bound methods.

Let’s understand this concept with an example:

# Python code to demonstrate
# use of bound methods
class A:
    def func(self, arg):
        self.arg = arg
        print("Value of arg = ", arg)
# Creating an instance
obj = A()  
# bound method


< bound method A.func of <__main__.A object at 0x7fb81c5a09e8>>


 obj.func(arg) is translated by python as A.func(obj, arg).

The instance obj is automatically passed as the first argument to the function called and hence the first parameter of the function will be used to access the variables/functions of the object.

Let’s see another example of the Bound method.

# Python code to demonstrate
# use of bound methods
class Car:
    # Car class created
    gears = 5
    # a class method to change the number of gears 
    def change_gears(cls, gears):
        cls.gears = gears
# instance of class Car created
Car1 = Car()
print("Car1 gears before calling change_gears() = ", Car1.gears)
print("Gears after calling change_gears() = ", Car1.gears)
# bound method


Car1 gears before calling change_gears() =  5
Gears after calling change_gears() =  6
<bound method Car.change_gears of <class '__main__.Car'>>

The above code is an example of a classmethod. A class method is like a bound method except that the class of the instance is passed as an argument rather than the instance itself. Here in the above example when we call Car1.change_gears(6), the class ‘Car’ is passed as the first argument.

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