Constructor (object-oriented programming): Difference between revisions

Revision as of 21:18, 3 August 2025

Revision as of 15:29, 4 August 2025

Line 545:

In the typical case, only the <code>__init__</code> method need be defined. (The most common exception is for immutable objects.)

<syntaxhighlight lang="pyconpython">

>>> class ExampleClass:

... def __new__(cls: type, value: int) -> 'ExampleClass':

... print("Creating new instance...")

... # Call the superclass constructor to create the instance.

... instance: 'ExampleClass' = super(ExampleClass, cls).__new__(cls)

... return instance

... def __init__(self, value: int) -> None:

... print("Initialising instance...")

... self.payload: int = value

>>> exampleInstance = ExampleClass(42)

if __name__ == "__main__":

>>> exampleInstance: ExampleClass = ExampleClass(42)

>>> print(exampleInstance.payload)

</syntaxhighlight>

This prints:

<pre>

Creating new instance...

Initialising instance...

>>> print(exampleInstance.payload)

42

</pre>

</syntaxhighlight>

Classes normally act as [[Factory (object-oriented programming)|factories]] for new instances of themselves, that is, a class is a callable object (like a function), with the call being the constructor, and calling the class returns an instance of that class. However the <code>__new__</code> method is permitted to return something other than an instance of the class for specialised purposes. In that case, the <code>__init__</code> is not invoked.<ref name="auto"/>