Why can't Python decorators be chained across definitions?

Question

Why arn't the following two scripts equivalent?

(Taken from another question: Understanding Python Decorators)

def makebold(fn):
    def wrapped():
        return "<b>" + fn() + "</b>"
    return wrapped

def makeitalic(fn):
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

@makebold
@makeitalic
def hello():
    return "hello world"

print hello() ## returns <b><i>hello world</i></b>

and with a decorated decorator:

def makebold(fn):
    def wrapped():
        return "<b>" + fn() + "</b>"
    return wrapped

@makebold
def makeitalic(fn):
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

@makeitalic
def hello():
    return "hello world"

print hello() ## TypeError: wrapped() takes no arguments (1 given)

Why do I want to know? I've written a retry decorator to catch MySQLdb exceptions - if the exception is transient (e.g. Timeout) it will re-call the function after sleeping a bit.

I've also got a modifies_db decorator which takes care of some cache-related housekeeping. modifies_db is decorated with retry, so I assumed that all functions decorated with modifies_db would also retry implicitly. Where did I go wrong?

Answer 1

The problem with the second example is that

@makebold
def makeitalic(fn):
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

is trying to decorate makeitalic, the decorator, and not wrapped, the function it returns.

You can do what I think you intend with something like this:

def makeitalic(fn):
    @makebold
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

Here makeitalic uses makebold to decorate wrapped.

Answer 2

The reason is because wrapped() inside of makebold doesn't accept any arguments.

When you use the decorator like that it can cause some issues, I'll post an example of how to achieve what you're wanting though, give me just a moment.

Here is a working example of what you need.

def makebold(rewrap=False):
    if rewrap:
        def inner(decorator):
            def rewrapper(func):
                def wrapped(*args, **kwargs):
                    return "<b>%s</b>" % decorator(func)(*args,**kwargs)
                return wrapped
            return rewrapper
        return inner

    else:
        def inner(func):
            def wrapped(*args, **kwargs):
                return "<b>%s</b>" % func(*args, **kwargs)    
            return wrapped
        return inner

@makebold(rewrap=True)
def makeitalic(fn):
    def wrapped(*args, **kwargs):
        return "<i>%s</i>" % fn(*args, **kwargs)
    return wrapped

@makeitalic
def hello():
    return "hello world"

@makebold()
def hello2():
    return "Bob Dole"    

if __name__ == "__main__":
    print hello()   
    print hello2()

makebold is kinda ugly, but it shows you how to write a decorator that can optionally wrap another decorator.

Here is the output from the above script:

<b><i>hello world</i></b>
<b>Bob Dole</b>

Note that makebold is the only recursive decorator. Also note the subtle difference in usage: @makebold() vs @makeitalic.

Answer 3

The problem is replacing "makeitalic" (which takes one argument) with the "wrapped"-function in "makebold" which takes zero arguments.

Use *args, **kwargs to pass on arguments further down the chain:

def wrapped(*args, **kwargs):
    return "<b>" + fn(*args, **kwargs) + "</b>"