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

You can also decorate the method at runtime, but not at define time. This could be useful in the case where you don't have access to or don't want to edit the source code, for example.

In [1]: class Toy():
   ...:     def __init__(self):
   ...:         return
   ...:     def shout(self, s):
   ...:         print(s)
   ...:

In [2]: def decor(fn):
   ...:     def wrapper(*args):
   ...:         print("I'm decorated")
   ...:         return fn(*args)
   ...:     return wrapper
   ...:


In [4]:

In [4]: a=Toy()

In [5]: a.shout('sa')
sa

In [6]: a.shout=decor(a.shout)

In [7]: a.shout('sa')
I'm decorated
sa

Python automatically passes the class instance as reference. (The self argument which is seen in all instance methods).

You could do:

def formatHeader(fn):
    def wrapped(self=None):
        return '<div class="page_header">'+fn(self)+'</div>'
    return wrapped