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
__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 you have a good factory example.