decorating decorators: try to get my head around understanding it

Question

I'm trying to understand how to decorate decorators, and wanted to try out the following:

Let's say I have two decorators and apply them to the function hello():

def wrap(f):     def wrapper():         return " ".join(f())     return wrapper   def upper(f):     def uppercase(*args, **kargs):         a,b = f(*args, **kargs)         return a.upper(), b.upper()     return uppercase  @wrap @upper def hello():     return "hello","world"  print(hello()) 

Then I have to start adding other decorators for other functions, but in general the @wrap decorator will "wrap all of them"

def lower(f):     def lowercase(*args, **kargs):         a,b = f(*args, **kargs)         return a.lower(), b.lower()     return lowercase  @wrap @lower def byebye():     return "bye", "bye" 

How do I write a decorator, which decorates my @lower and @upper decorators? See below:

@wrap def lower():     ...  @wrap def upper():     ... 

To achieve the same result as above by only doing:

@upper def hello():     ...  @lower def byebye():     ... 

Answer 1

def upper(f):     @wrap     def uppercase(*args, **kargs):         a,b = f(*args, **kargs)         return a.upper(), b.upper()     return uppercase 

---

A decorator in Python

 @foo  def bar(...): ... 

is just equivalent to

 def bar(...): ...  bar = foo(bar) 

You want to get the effect of

@wrap @upper def hello(): .... 

i.e.

hello = wrap(upper(hello)) 

so the wrap should be called on the return value of upper:

def upper_with_wrap(f):    def uppercase(...): ...    return wrap(uppercase) 

which is also equivalent to applying the decorator on that function:

def upper_with_wrap(f):    @wrap    def uppercase(...): ...    # ^ equivalent to 'uppercase = wrap(uppercase)'    return uppercase 

Answer 2

Here's a generic (and slightly convoluted) solution for decorating decorators with decorators (Yay!).

# A second-order decorator def decdec(inner_dec):     def ddmain(outer_dec):         def decwrapper(f):             wrapped = inner_dec(outer_dec(f))             def fwrapper(*args, **kwargs):                return wrapped(*args, **kwargs)             return fwrapper         return decwrapper     return ddmain  def wrap(f):     def wrapper():         return " ".join(f())     return wrapper   # Decorate upper (a decorator) with wrap (another decorator) @decdec(wrap) def upper(f):     def uppercase(*args, **kargs):         a,b = f(*args, **kargs)         return a.upper(), b.upper()     return uppercase  @upper def hello():     return "hello","world"  print(hello())