Editing Constructor (object-oriented programming) (section)

=== Python ===
In [[Python (programming language)|Python]], constructors are defined by one or both of <code>__new__</code> and <code>__init__</code> methods. A new instance is created by calling the class as if it were a function, which calls the <code>__new__</code> and <code>__init__</code> methods. If a constructor method is not defined in the class, the next one found in the class's Method Resolution Order will be called.<ref name="auto">{{Cite web|url=https://docs.python.org/3/reference/datamodel.html|title=3. Data model|website=Python documentation}}</ref>

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

<syntaxhighlight lang="pycon">
>>> class ExampleClass:
...     def __new__(cls, value):
...         print("Creating new instance...")
...         # Call the superclass constructor to create the instance.
...         instance = super(ExampleClass, cls).__new__(cls)
...         return instance
...     def __init__(self, value):
...         print("Initialising instance...")
...         self.payload = value
>>> exampleInstance = ExampleClass(42)
Creating new instance...
Initialising instance...
>>> print(exampleInstance.payload)
42
</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"/>