How to decorate a method inside a class?

You are omitting the self parameter which is present in the undecorated function (createHeader in your case).

def formatHeader(fn):
    from functools import wraps
    @wraps(fn)
    def wrapper(self):
        return '<div class="page_header">'+fn(self)+'</div>'
    return wrapper

If you are unsure about the signature of the function you want to decorate, you can make it rather general as follows:

def formatHeader(fn):
    from functools import wraps
    @wraps(fn)
    def wrapper(*args, **kw):
        return '<div class="page_header">'+fn(*args, **kw)+'</div>'
    return wrapper