How to cast in python

Question

Hi

I have a doubt on casting in python.

Refer below code.

How can I cast for test1 to test?

a = 1.3
b = int(a)

Works fine, but it is as both are compatible.

In my case, test1 and test are not compatible , but it should give some error related to incompatible type rather than argument errors. Right? Or am I missing something?
https://sololearn.com/compiler-playground/cPSVLCwhI2Pn/?ref=app

Answer 1

How can we speak in terms of casting if python is not a strongly-typed language like Java etc? It makes no sense to me.

Answer 2

No, this will give an argument error. The test __init__ method doesn’t take any arguments, let alone an instance of test11. test11 isn’t quite “incompatible” with test per se as there’s no way for them to interact in the first place.

Answer 3

Casting in python is not possible(necessary). A variable in Python can change its type.
A="test"
A=3

Is possible.

Answer 4

If you want a new instance of Class 2 to use data from an instance of Class 1, you must set up Class 2’s __init__ method to accept an instance of Class 1 as an argument and set Class 2’s attributes based on it.

It might help if you said exactly what you were trying to do. You may have condensed the question incorrectly.

Answer 5

Ketan Lalcheta
Building on Wilbur Jaywright's comment, you can define casting functions.

Don't mix in inheritance, but the classes have to be compatible, though, so that the attributes can be passed on.

Without the inheritance complication, casting becomes just a simple re-instancing to a different class and passing and possibly recasting the attributes.

https://sololearn.com/compiler-playground/civfWz7riwrT/?ref=app

Answer 6

Oma Falk
yes, Python's dynamic typing makes it really easy to switch classes. So just re-initialze the variable as something else.

Ketan Lalcheta 's condition of not wanting to reinstantiate with __init__ is a hard obstacle, since passing by value is the default mode of getting things done in Python.

Answer 7

Bob_Li yeah but thats a mantra of OOP. We need __init__ to get the initial state. All called methods rely on it.
ABAP (SAP-language) can do it: A variable with type superclass can be assigned to inhereted class. But you MUST declare that you know what you do. Normally this is a strong hint for a bad style.... and you dont know what you do :-)
On the other hand if class B inherits from classA: a B-typed variable can can be assigned to a variable of Class A. Which is finally polymorphism.

Answer 8

It gives an argument error, since you are giving “arguments” to the init function / to the class. When using “test(someObj)” you are passing the someObj variable to the __init__ function. The first argument will always be “self” for non-static functions

Answer 9

Sounds good. I was under impression of c++ and hence getting confused.

I buy this argument and accept that derived class object can never be behaved as base class object

Answer 10

I saw the code. in the second last line you wrote "obj2 = test(obj1)" . the problem is that you gave 2 arguments to the class "test", one is self(by default) and another is obj1 ,but your class takes only one argument which is self by default. so don't give any argument to test. i hope your doubt is cleared

Answer 11

Then type cast in python for classes is applicable only to derived classes ?

How to do type change in case of inheritance? Is it used to get derived from base or base from derived ? And how ?

Answer 12

I Don't want to reuse the variable. I need to behave A as some other type

For example,

b = 3.3
Int(b) will behave as int inspite 3.3 was decimal

Answer 13

In other words, can I get base class method invoked from y or derived class method invoked from x in attached code ?
https://sololearn.com/compiler-playground/cLI3Si5x13Hh/?ref=app

Answer 14

A is animal and B is cat for example.

And it is like cat(animal) for Inheritance hierarchy.

Now, if I have objCat = Cat() how should I make objCat behave as animal? I don't want to create a new object using __init__ which takes cat object as argument for animal Class.

I need objCat to behave as if it is animal class object which is parent of cat.