Class decorator not being called everytime

Question

A seemingly easy thing which i cant get around.

registry  =  {}

def register(cls):
    registry[cls.__clsid__] = cls
    print cls
    return cls

@register
class Foo(object):
    __clsid__ = "123-456"

    def bar(self):
        pass
c=Foo()
d=Foo()

e=Foo()

Output:

<class '__main__.Foo'>

Now i expect decorator to be called 3 times.Why has it been called only once.

So what are you trying to achieve here? A class object is created just once; you are not creating more classes, you are creating *instances*. Those `Foo()` calls don't re-execute the `class` statement. — Martijn Pieters, Oct 20 '16 at 13:46

Martijn Pieters · Accepted Answer · 2016-10-20T13:50:04.160

4

A class decorator is applied when the class is created, not each time an instance is created.

The @register line applies to the class Foo(object): statement only. This is run just once, when the module is imported.

Creating an instance does not need to re-run the class statement because instances are just objects that keep a reference to the class (type(c) returns the Foo class object); instances are not 'copies' of a class object.

If you want to register instances you'll either have to do so in the __init__ or the __new__ method of a class (which can be decorated too). __new__ is responsible for creating the instance, __init__ is the hook called to initialise that instance.

edited Oct 20 '16 at 13:50

answered Oct 20 '16 at 13:44

Martijn Pieters

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But isnt `class` just a template which comes into existence when `object` is created – vks Oct 20 '16 at 13:47
1

@vks: no, a class is an object. Calling that object then produces an instance which keeps a reference to the class. Through that reference attributes are resolved, which is how methods are found, for example. – Martijn Pieters Oct 20 '16 at 13:48
hey http://stackoverflow.com/questions/6760685/creating-a-singleton-in-python we have singleton decorator for class....which allows only one object...if decorator is called only once,then how does it manage this? – vks Nov 08 '16 at 15:11
The class decorator in that question doesn't return a class object. It returns a function instead. You don't *have* to return a class object from a class decorator, at all. You just have to return *something* that'll then be bound to the class name. – Martijn Pieters Nov 08 '16 at 15:15
ok but still when i call for new object it wont get called right? then how it is this getting handled? – vks Nov 08 '16 at 15:20
Please do read the code of the class decorator; the function that's returned would be called instead, and it is responsible for creating an instance (and it'll only create *one*). – Martijn Pieters Nov 08 '16 at 15:24

score 2 · Answer 2 · answered Oct 20 '16 at 13:44

2

The class decorator is being applied to the class itself, and it is applied only once, when the class is defined. Basically, it processes the class definition and produces a new class.

So you'd only process it once.

answered Oct 20 '16 at 13:44

Horia Coman

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Class decorator not being called everytime

2 Answers2