How can i perform constructor overloading in python?

Question

I have an employee class which I would like to fill in two different ways, and would like to know do this by constructor overloading

class Employee:

    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.pay = pay

    def __init__(self, data):
        (self.first, self.last, self.pay) = data

This is because, either I have to initialise the class like

Employee('John','Smith',3000)

Or I would like to initialise the class by passing a tuple like

data = ('John','Smith',3000)
Employee(data)

There is no constructor/method/function overloading in Python. You can use either optional keyword-parameters, or decide what to do based on the type of the parameter. For this specific case, however, you can just do Employee(*data), unpacking the tuple to the individual parameters. — tobias_k
– tobias_k, Commented Nov 14, 2019 at 14:04
If instead of Employee(data) you used Employee(*data), you could use the single, straight-forward constructor. — Scott Hunter
– Scott Hunter, Commented Nov 14, 2019 at 14:05

chepner · Accepted Answer · 2019-11-14 14:07:09Z

You don't, really. You could mess around with a definition like

def __init__(self, *args):

then do lots of processing that checks the number of arguments, the type of the first argument, etc. Or, you can simply be explicit and define a separate constructor whose name describes exactly what it does. The class method can do some validation on the tuple before passing its contents to the default constructor.

class Employee:
    def __init__(self, first, last, pay):
        self.first = first
        self.last = last
        self.pay = pay

    @classmethod
    def from_tuple(cls, t):
        if len(t) != 3:
            raise ValueError("Wrong number of items in the tuple")

        return cls(*t)

data = ('John','Smith',3000)
Employee.from_tuple(data)

Of course, the simplicity of the class method's definition suggests that you don't need to go to this much trouble: if you know the tuple has the right number and kinds of values, just unpack data for use with the default constructor.

Employee(*data)

By "default constructor", I mean the method which Employee.__new__ resolves to. Employee.__init__ is technically an initializer, invoked when appropriate on an already constructed instance returned by __new__.

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