Python Method Definition Using Decorators

Question

Can someone thoroughly explain the last line of the following code:

def myMethod(self):
    # do something

myMethod = transformMethod(myMethod)

Why would you want to pass the definition for a method through another method? And how would that even work? Thanks in advance!

Answer 1

This is an example of function wrapping, which is when you have a function that accepts a function as an argument, and returns a new function that modifies the behavior of the original function.

Here is an example of how this might be used, this is a simple wrapper which just prints 'Enter' and 'Exit' on each call:

def wrapper(func):
    def wrapped():
        print 'Enter'
        result = func()
        print 'Exit'
        return result
    return wrapped

And here is an example of how you could use this:

>>> def say_hello():
...     print 'Hello'
... 
>>> say_hello()  # behavior before wrapping
Hello
>>> say_hello = wrapper(say_hello)
>>> say_hello()  # behavior after wrapping
Enter
Hello
Exit

For convenience, Python provides the decorator syntax which is just a shorthand version of function wrapping that does the same thing at function definition time, here is how this can be used:

>>> @wrapper
... def say_hello():
...     print 'Hello'
... 
>>> say_hello()
Enter
Hello
Exit

Answer 2

Why would you want to pass the definition for a method through another method?

Because you want to modify its behavior.

And how would that even work?

Perfectly, since functions are first-class in Python.

def decorator(f):
  def wrapper(*args, **kwargs):
    print 'Before!'
    res = f(*args, **kwargs)
    print 'After!'
    return res
  return wrapper

def somemethod():
  print 'During...'

somemethod = decorator(somemethod)

somemethod()