What is a clean, Pythonic way to have multiple constructors in Python?

29 Dec, 2020
Prerequisite – Constructors, @classmethod decorators
Python does not support explicit multiple constructors, yet there are some ways using which the multiple constructors can be achieved. If multiple __init__ methods are written for the same class, then the latest one overwrites all the previous constructors. Look at the example below.
Python3
class example:      def __init__(self):        print("One")      def __init__(self):        print("Two")      def __init__(self):        print("Three")    e = example()
Three

Multiple constructors are required when one has to perform different actions on the instantiation of a class. This is useful when the class has to perform different actions on different parameters. The class constructors can be made to exhibit polymorphism in three ways which are listed below.
Overloading constructors based on arguments.Calling methods from __init__.Using @classmethod decorator.
Overloading constructors based on arguments.
Calling methods from __init__.
Using @classmethod decorator.
This article explains how to have multiple constructors in a clean and Pythonic way with examples.
The constructor overloading is done by checking conditions for the arguments passed and performing required actions. For example, consider passing an argument to the class sample, 
If the parameter is an int, the square of the number should be the answer.
If the parameter is a String, the answer should be “Hello!!”+string.
If the parameter is of length greater than 1, the sum of arguments should be stored as the answer.
Python3
class sample:      # constructor overloading    # based on args    def __init__(self, *args):          # if args are more than 1        # sum of args        if len(args) > 1:            self.ans = 0            for i in args:                self.ans += i          # if arg is an integer        # square the arg        elif isinstance(args[0], int):            self.ans = args[0]*args[0]          # if arg is string        # Print with hello        elif isinstance(args[0], str):            self.ans = "Hello! "+args[0]+"."    s1 = sample(1, 2, 3, 4, 5)print("Sum of list :", s1.ans)  s2 = sample(5)print("Square of int :", s2.ans)  s3 = sample("GeeksforGeeks")print("String :", s3.ans)
Sum of list : 15
Square of int : 25
String : Hello! GeeksforGeeks.

In the code above, the instance variable was ans, but its values differ based on the arguments. Since a variable number of arguments for the class, *args is used which is a tuple that contains the arguments passed and can be accessed using an index. In the case of int and string, only one argument is passed and thus accessed as args[0] (the only element in the tuple).
A class can have one constructor __init__ which can perform any action when the instance of the class is created. This constructor can be made to different functions that carry out different actions based on the arguments passed. Now consider an example : 
If the number of arguments passed is 2, then evaluate the expression x = a2-b2
If the number of arguments passed is 3, then evaluate the expression y = a2+b2-c.
If more than 3 arguments have been passed, then sum up the squares, divide it by the highest value in the arguments passed.
Python3
class eval_equations:    # single constructor to call other methods    def __init__(self, *inp):          # when 2 arguments are passed        if len(inp) == 2:            self.ans = self.eq2(inp)          # when 3 arguments are passed        elif len(inp) == 3:            self.ans = self.eq1(inp)          # when more than 3 arguments are passed        else:            self.ans = self.eq3(inp)      def eq1(self, args):        x = (args[0]*args[0])+(args[1]*args[1])-args[2]        return x      def eq2(self, args):        y = (args[0]*args[0])-(args[1]*args[1])        return y      def eq3(self, args):        temp = 0        for i in range(0, len(args)):            temp += args[i]*args[i]                  temp = temp/max(args)        z = temp        return z    inp1 = eval_equations(1, 2)inp2 = eval_equations(1, 2, 3)inp3 = eval_equations(1, 2, 3, 4, 5)  print("equation 2 :", inp1.ans)print("equation 1 :", inp2.ans)print("equation 3 :", inp3.ans)
equation 2 : -3
equation 1 : 2
equation 3 : 11.0

 In the example above, the equation to be evaluated is written on different instance methods and made to return the answer. The constructor calls the appropriate method and acts differently for different parameters.
The expressions have been evaluated as follows:
inputs : 1,2 —> 12-22 = 1-4 = -3
inputs : 1,2,3  —> (12 + 22) – 3 = 5-3 = 2
inputs : 1,2,3,4,5 —> (12 + 22 + 32 + 42 + 52) / 5 = 55/5 = 11.0
This decorator allows a function to be accessible without instantiating the class. The functions can be accessed both by the instance of the class and the class itself. The first parameter of the method that is declared as classmethod is cls, which is like the self of the instance methods. Here cls refer to the class itself. This proves to be very helpful to use multiple constructors in Python and is a more Pythonic approach considered to the above ones. Consider the same example used above. Evaluate different expressions based on the number of inputs.
Python3
class eval_equations:      # basic constructor    def __init__(self, a):        self.ans = a      # expression 1    @classmethod    def eq1(cls, args):              # create an object for the class to return        x = cls((args[0]*args[0])+(args[1]*args[1])-args[2])        return x      # expression 2    @classmethod    def eq2(cls, args):        y = cls((args[0]*args[0])-(args[1]*args[1]))        return y      # expression 3    @classmethod    def eq3(cls, args):        temp = 0          # square of each element        for i in range(0, len(args)):            temp += args[i]*args[i]          temp = temp/max(args)        z = cls(temp)        return z    li = [[1, 2], [1, 2, 3], [1, 2, 3, 4, 5]]i = 0  # loop to get input three timeswhile i < 3:      inp = li[i]      # no.of.arguments = 2    if len(inp) == 2:        p = eval_equations.eq2(inp)        print("equation 2 :", p.ans)      # no.of.arguments = 3    elif len(inp) == 3:        p = eval_equations.eq1(inp)        print("equation 1 :", p.ans)      # More than three arguments    else:        p = eval_equations.eq3(inp)        print("equation 3 :", p.ans)      #increment loop            i += 1
equation 2 : -3
equation 1 : 2
equation 3 : 11.0

In the example above, the instance of the object is not created initially. The class methods to evaluate various expression has been defined with @classmethod decorator. Now they can be called with the class name and the object is created in that class method after evaluating the expression. The instance variable holds different answers for a different number of parameters passed.
Python Oops-programs
Python-OOP
python-oop-concepts
Python
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.