How to use a callback function in python?

Question

I wonder how to correctly use python 2.7 callback functions.

I have some callback functions from Cherrypy auth examples in my code.

(These callbacks return a function that can evaluate to True or False, depending on the logged in user being in a group or not.)

I wonder if a callback is executed or not if I write a piece of code like this:

Given the definition from the library is:

def member_of(groupname):
    def check():
        if groupname == 'admin':
          if cherrypy.request.login == 'joe':
            return True
          if cherrypy.request.login == 'toni':
            return True
          return False
        return False
        # .... (other groups checked in the same way)
    return check # returns a callback function from my understanding?

How can I apply and execute the callback in my code?

If I put it like this:

 if member_of('admin'):
    do_something()
  else:
    do_something_else()

Will this execute the calllback and check for the admin group? Or will it find out if the value of "member_of" is a function definition and a function definition is probably always a "True" value (or maybe a False value) but both are wrong, because it needs to be executed

Can you enlighten me on this? How can I make sure a callback is executed? An how can I pass it around as it is?

Answer 1

If you execute it, it is plain simple.

member_of() will return method object check. you have to execute to get result by doing something like if member_of('admin')(): or,

k=member_of('admin')
if k():

To do your task.

Answer 2

In python, like in many other languages, a variable can also contain a function and you can pass them around like other variables that contain e.g. numbers or strings.

CherryPy's member_of function itself does return a function in your example.

I am explaining it in simple steps:

If you write member_of() it returns the result of the function member_of() which is the function with the name check in this case.

cb_function = member_of('admin')

At this point the variable cb_function holds the result of calling the function member_of, and in the last line member_of returns check, which was defined within the function member_of as another function!

You have to call the first result again, because you can and you have to treat it in almost the same way as a local function, that you defined in the current context, to get the final result, by doing something like:

my_result =  cb_function()

And then you would continue and use the result. For example you could check its boolean value:

if my_result:
  # do something
  ...   

The 3 steps from above together can be written shorter:

cb_function = member_of('admin')
  if cb_function():
    # do something
    ...

Or even shorter:

if member_of('admin')():
  # do something
  ...  

At first it may appear a little strange in python to have the double ()(), but if you think about it for a while it makes sense.