CODEWITHSUNDEEP
pythonpython-3.xpython-internals
ThinkGeek
ThinkGeek

Reputation: 5117

object takes no parameters after defining __new__

In python 3.3 and later, when we override __new__(), we don't have to pass arguments and keyword arguments to super().__new__() or object.__new__(). But this call to super().__new__() returns an instance of a class.

How does python pass the rest of the arguments to __init__ then?

class Spam(object):
    ''' Hello I am Spam '''

    def __new__(cls, *args, **kwargs):
        print("Creating Instance")
        instance = object.__new__(cls) # Who passed *args and **kwargs to __init__?
        print(instance)
        return instance

    def __init__(self, a, b):
        print("Init Called")
        self.a = a
        self.b = b

Can someone please explain whats happening here?

Upvotes: 2

Views: 1309

Answers (2)

CristiFati
CristiFati

Reputation: 41116

You passed cls as an argument to object.__new__, so the interpreter can check whether instance is an instance of cls.

The initializer (__init__) is automatically called by the allocator (__new__) as [Python.Docs]: object.__new__(cls[, ...]) states (emphases are mine):

If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__().

If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked.

code00.py:

#!/usr/bin/env python3

import sys


class Spam(object):
    ''' Hello I am Spam '''

    def __new__(cls, *args, **kwargs):
        print("Creating Instance")
        instance = object.__new__(cls) # Who passed *args and **kwargs to __init__?
        print(instance)
        #return instance  # If you return anything else (1, object(), or None by commenting the line) here, __init__ won't be called
        if len(sys.argv) == 1:  # DO NOT DO THIS!!! It's just for demo purposes
            return instance


    def __init__(self, a, b):
        print("Init Called")
        self.a = a
        self.b = b


def main():
    spam = Spam(1, 2)
    print(type(spam), dir(spam))


if __name__ == "__main__":
    print("Python {0:s} on {0:s}\n".format(sys.version, sys.platform))
    main()

Output:

e:\Work\Dev\StackOverflow\q054511671>"e:\Work\Dev\VEnvs\py_064_03.06.08_test0\Scripts\python.exe" code00.py
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32

Creating Instance
<__main__.Spam object at 0x000001F8E24D14E0>
Init Called
<class '__main__.Spam'> ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'a', 'b']

e:\Work\Dev\StackOverflow\q054511671>"e:\Work\Dev\VEnvs\py_064_03.06.08_test0\Scripts\python.exe" code.py arg
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32

Creating Instance
<__main__.Spam object at 0x0000020808F71550>
<class 'NoneType'> ['__bool__', '__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

Note that this isn't specific to Python 3 (check [Python 2.Docs]: Data model), but rather to [Python]: New-style Classes For more details, you could also check check [Python 2.2.Docs]: Overriding the __new__ method (the Singleton class).

Upvotes: 2

Serge Ballesta
Serge Ballesta

Reputation: 148890

What matters here is the initial call, say spam = Spam('x', 1).

Internally, Python calls __new__ as a class method on class Spam with the passed parameters. What Spam.__new__ actually does is not really important, it is just supposed to return an object.

It does use object.__new__ to build a Spam object. As the created object has the correct class, Python calls __init__ on it with the initial parameters.

Upvotes: 1

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