When does class level decorator decorates?

Question

I have created a decorator to decorate all instance methods of a class. I have written following code to do so.

def debug(func):
    msg = func.__name__

    @wraps(func)
    def wrapper(*args, **kwargs):
        print(msg)
        return func(*args, **kwargs)
    return wrapper 

# Decorator for all the methods in a class
def debugmethods(cls):
    for key, val in vars(cls).items():
        if callable(val):
            setattr(cls, key, debug(val))
    return cls

@debugmethods
class Spam:
    def foo(self):
        pass

    def bar(self):
        pass

Now I am trying to understand how this works, I mean when will this decoration happen and how can I check that?

a) It already happened?

b) When I access Spam class for the first time? For e.g.

for key, val in Spam.__dict__.items():
        print(key, val)

c) When I instantiate Spam class for the first time? For e.g.

 spam = Spam()
 for key, val in Spam.__dict__.items():
     print(key, val)

Answer 1

This is actually easily seen in action if you add a few print lines:

print('Deocrator is being defined')
def deco(cls):
    print('Decorator is called')
    cls.decorated = True
    return cls

print('Foo is being defined')
@deco
class Foo:
    print('Foo class attributes are being set')
    def __init__(self):
        print('Foo is being instantiated')

print('Foo class is being referenced in main script')
print(f'Foo is decorated: {Foo.decorated}')
print('Foo instance is being created in main script')
print(Foo())

Result:

Deocrator is being defined
Foo is being defined
Foo class attributes are being set
Decorator is called    # <--- decoration happens immediately after definition of Foo
Foo class is being referenced in main script
Foo is decorated: True
Foo instance is being created in main script
Foo is being instantiated
<__main__.Foo object at 0x00789150>

In short, the answer is much like what @jonrsharpe said, it already happened. Just figure the visual would help.