Python call __init__() method only one time

Question

Is there any way to call the init () method of a class only one time. Or how can I disable calling of init () when I create an object from a class?

Answer 1

If you want a singleton , where there is only ever one instance of a class then you can create a decorator like the following as gotten from PEP18 :

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class MyClass:
    pass

Try it out:

>>> a = MyClass()
>>> b = MyClass()
>>> a == b
True

Answer 2

There's no direct way to disable __init__ , but there are a few ways to work around this. One of them is having a flag:

class Class:
    _init_already = False
    __init__(self):
        if not Class._init_already:
            ...
            Class._init_already = True

But this is ugly. What is it that you are really trying to accomplish?

Answer 3

You shouldn't put anything in __init__() that you only want to run once. Each time you create an instance of a class __init__() will be run.

If you want to customize the creation of your class you should look into creating a metaclass . Basically, this lets you define a function that is only run once when the class is first defined.

Answer 4

The init method will always be called however you could create another method called run() or something, that you call after creating the object, optionally.

foo = ClassName()
foo.run() # replacement for __init__

foo2 = ClassName()