constructor overloading python

Question

I was trying to implement method overloading in Python, so I create a Class named Tes where I have two variables, one is va, other is vb.

class Tes:
    def __init__(self,a,b):
        self.va=a
        self.vb=b
        print(self.va)
        print(self.vb)
    def __init__(self,a):
        self.__init__(a,"b-default")

Tes("n1","n2")
Tes("n3")

But it gives me error in Tes("n1","n2"),saying : Tes("n1","n2") TypeError: __init__() takes 2 positional arguments but 3 were given

How to implement constructor overloading correctly in python?

Joran Beasley · Accepted Answer · 2019-12-08 04:32:57Z

2

I guess you are asking this

class Tes:
    def __init__(self,a,b="default_value_b"):
        self.va=a
        self.vb=b
        print(self.va)
        print(self.vb)

answered Dec 8, 2019 at 4:32

Joran Beasley

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1 Comment

Maifee Ul Asad Over a year ago

I can accept your answer in 2 minutes, till then can you take a look at - stackoverflow.com/questions/59232341/…

Liang Fan · Accepted Answer · 2019-12-08 04:37:59Z

2

You can only use overriding, rather than overloading.
This way may be useful.

def __init__(self, a, b="some value"):
    self.va = a
    self.vb = b
    # do something

answered Dec 8, 2019 at 4:37

Liang Fan

814 bronze badges

Comments

Hugo G · Accepted Answer · 2019-12-08 04:44:42Z

How to implement constructor overloading correctly in python?

Python does NOT support any constructor or function overloading. However, it doesn't need to. Instead, it supports setting default parameters.

class Tes:
    def __init__(self, a, b="default_value_b"):
        self.va = a
        self.vb = b
        print(self.va)
        print(self.vb)

The question changes a little if you are using typing. Typing is a relatively new to Python and purely optional. Typing allows you to statically validate your code against expectations, and it can ease writing code by providing live assistance.

In your simple case, you would not use overloading, because it only defines an optional parameter. The type of the parameter stays the same regardless. With Mypy type annotations, your code can be written as follows:


class Tes:
  def __init__(self, a: str, b: str = "default value for b") -> None:
    self.va = a
    self.vb = b
    print(self.va)
    print(self.vb)

If you want to allow None as a value for b, change the signature to:

def __init__(self, a: str, b: Optional[str] = "default value for b") -> None:

Optional can be imported from typing:

from typing import Optional

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