Bound, unbound, and static methods in Python

Methods in Python are like functions except that it is attached to an object.The methods are called on objects and it possibly make changes to that object. These methods can be Bound, Unbound or Static method. The static methods are one of the types of Unbound method. These types are explained in detail below.

Bound methods

If a function is an attribute of class and it is accessed via the instances, they are called bound methods. A bound method is one that has ‘self‘ as its first argument. Since these are dependent on the instance of classes, these are also known as instance methods.

Need for these bound methods

The methods inside the classes would take at least one argument. To make them zero-argument methods, ‘decorators‘ has to be used. Different instances of a class have different values associated with them.

For example, if there is a class “Fruits”, and instances like apple, orange, mango are possible. Each instance may have different size, color, taste, and nutrients in it. Thus to alter any value for a specific instance, the method must have ‘self’ as an argument that allows it to alter only its property.

Example:



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class sample(object):
  
    # Static variable for object number
    objectNo = 0
  
    def __init__(self, name1):
  
        # variable to hold name
        self.name = name1
  
        # Increment static variable for each object
        sample.objectNo = sample.objectNo + 1
  
        # each object's unique number that can be
        # considered as ID
        self.objNumber = sample.objectNo
  
    def myFunc(self):
        print("My name is ", self.name, 
              "from object ", self.objNumber)
  
    def alterIt(self, newName):
        self.name = newName
  
    def myFunc2():
        print("I am not a bound method !!!")
  
  
# creating first instance of class sample        
samp1 = sample("A")
samp1.myFunc()
  
# unhide the line below to see the error
# samp1.myFunc2() #----------> error line
  
  
# creating second instance of class sample    
samp2 = sample("B")
samp2.myFunc()
samp2.alterIt("C")
samp2.myFunc()
samp1.myFunc()

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Output:

My name is  A from object  1
My name is  B from object  2
My name is  C from object  2
My name is  A from object  1

In the above example two instances namely samp1 and samp2 are created. Note that when the function alterIt() is applied to the second instance, only that particular instance’s value is changed. The line samp1.myFunc() will be expanded as sample.myFunc(samp1). For this method no explicit argument is required to be passed. The instance samp1 will be passed as argument to the myFunc(). The line samp1.myFunc2() will generate the error :

Traceback (most recent call last):
  File "/home/4f130d34a1a72402e0d26bab554c2cf6.py", line 26, in 
    samp1.myFunc2() #----------> error line
TypeError: myFunc2() takes 0 positional arguments but 1 was given

It means that this method is unbound. It does not accept any instance as an argument. These functions are unbound functions.

Unbound methods and Static methods

Methods that do not have an instance of the class as the first argument are known as unbound methods. As of Python 3.0, the unbound methods have been removed from the language. They are not bounded with any specific object of the class. To make the method myFunc2() work in the above class it should be made into a static method

Static methods are similar to class methods but are bound completely to class instead of particular objects. They are accessed using class names.

Need for making a method static

Not all the methods need to alter the instances of a class. They might serve any common purpose. A method may be a utility function also.

For example, When we need to use certain mathematical functions, we use the built in class Math. The methods in this class are made static because they have nothing to do with specific objects. They do common actions. Thus each time it is not an optimized way to write as:

math=Math()
math.ceil(5.23)

So they can be simply accessed using their class name as Math.ceil(5.23).



A method can be made static in two ways:

  1. Using staticmethod()
  2. Using decorator

Using staticmethod(): The staticmethod() function takes the function to be converted as its argument and returns the static version of that function. A static function knows nothing about the class, it just works with the parameters passed to it.

Example:

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class sample():
       
      def myFunc2(x):
     
             print("I am", x, "from the static method")
  
sample.myFunc2 = staticmethod(sample.myFunc2)
sample.myFunc2("A")

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Output:

I am A from the static method

Using decorators: These are features of Python used for modifying one part of the program using another part of the program at the time of compilation. The decorator that can be used to make a method static is

@staticmethod

This informs the built-in default metaclass not to create any bound methods for this method. Once this line is added before a function, the function can be called using the class name. Consider the example taken for the Bound method where we encountered an error. To overcome that, it can be written as:

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class sample(object):
  
    # Static variable for object number
    objectNo = 0
  
    def __init__(self, name1):
        # variable to hold name
        self.name = name1
  
        # Increment static variable for each object
        sample.objectNo = sample.objectNo + 1
  
        # each object's unique number that can be
        # considered as ID
        self.objNumber = sample.objectNo
  
    def myFunc(self):
        print("My name is ", self.name, 
              "from object ", self.objNumber)
  
    def alterIt(self, newName):
        self.name = newName
  
    # using decorator to make the method static
    @staticmethod
    def myFunc2():
        print("I am not a bound method !!!")
  
  
# creating first instance of class sample        
samp1 = sample("A")
samp1.myFunc()
  
  
sample.myFunc2() #----------> error line
  
  
# creating second instance of class sample    
samp2 = sample("B")
samp2.myFunc()
samp2.alterIt("C")
samp2.myFunc()
samp1.myFunc()

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Output:

My name is  A from object  1
I am not a bound method !!!
My name is  B from object  2
My name is  C from object  2
My name is  A from object  1

Here, the line sample.myFunc2() runs without any error and the print() within it works perfectly and gives the output I am not a bound method!!!




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