I'm just trying to streamline one of my classes and have introduced some functionality in the same style as the flyweight design pattern.
However, I'm a bit confused as to why __init__
is always called after __new__
. I wasn't expecting this. Can anyone tell me why this is happening and how I can implement this functionality otherwise? (Apart from putting the implementation into the __new__
which feels quite hacky.)
Here's an example:
class A(object): _dict = dict() def __new__(cls): if 'key' in A._dict: print "EXISTS" return A._dict['key'] else: print "NEW" return super(A, cls).__new__(cls) def __init__(self): print "INIT" A._dict['key'] = self print "" a1 = A() a2 = A() a3 = A()
Outputs:
NEW INIT EXISTS INIT EXISTS INIT
Why?
Use the __init__() method to initialize the object's attributes. The __init__() doesn't create an object but is automatically called after the object is created.
The __init__ function is called every time an object is created from a class. The __init__ method lets the class initialize the object's attributes and serves no other purpose.
Init functions are called only once, after all the variable declarations and before the main function. It allows you to initialize whatever your program needs before running.
The __new__() is a static method of the object class. When you create a new object by calling the class, Python calls the __new__() method to create the object first and then calls the __init__() method to initialize the object's attributes.
Use
__new__
when you need to control the creation of a new instance.
Use
__init__
when you need to control initialization of a new instance.
__new__
is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class.
In contrast,
__init__
doesn't return anything; it's only responsible for initializing the instance after it's been created.In general, you shouldn't need to override
__new__
unless you're subclassing an immutable type like str, int, unicode or tuple.
From April 2008 post: When to use __new__
vs. __init__
? on mail.python.org.
You should consider that what you are trying to do is usually done with a Factory and that's the best way to do it. Using __new__
is not a good clean solution so please consider the usage of a factory. Here's a good example: ActiveState Fᴀᴄᴛᴏʀʏ ᴘᴀᴛᴛᴇʀɴ Recipe.
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