decorator_with_state.py

__foo = None

class Foo:
    def __init__(self, x):
        self.x = x

def init_global_foo(x):
    global __foo
    __foo = Foo(x)

def with_foo(foo=None):

    def decorator(func):
        def inject(foo):
            def wrapper(*args, **kwargs):
                # this wrapper could do anything at this point
                # but here's a simple arg injection example
                return func(*args, **kwargs, x=foo.x)
            return wrapper

        global __foo
        if foo is not None:
            return inject(foo)
        elif __foo is not None:
            return inject(__foo)
        else:
            raise ValueError("foo is undefined")

    return decorator


if __name__ == "__main__":

    # decorate with a init once style
    init_global_foo(99)

    @with_foo()
    def f(x):
        print(x)

    # decorate with a init on decorate style
    @with_foo(Foo(42))
    def g(x):
        print(x)

    @with_foo(Foo(43))
    def h(x):
        print(x)

    f()
    g()
    h()