color version - PET: Python Entre Todos - Python Argentina

id(greeting)
3068236156L
>>> greeting.__name__
'greeting'
>>> say_hello = greeting
>>> say_hello()
hello
Decorator (non-strict definition)
Let’s take a liberty for a moment and say that a decorator is a function d that receives as
parameter another function a and returns a new function r.
Code
def d(a):
def r(*args, **kwargs):
print "Start of execution of", a.__name__
a(*args, **kwargs)
print "End of execution of", a.__name__
return r
When executing a decorated function with the above decorator, a bit of text will be
shown, then the decorated function will be executed and more text will appear as it
finishes.
In sum2 we keep the decorated version of sum. Now see what happens when we execute
it:
• d: function decorator
• a: function to decorate
• r: function decorated
We can apply the decorator using a functional notation:
a = d(a)
Let’s see how to implement a generic decorator:
Code
def d(a):
def r(*args, **kwargs):
# a's previous execution behavior
a(*args, **kwargs)
# a's after execution behavior
return r
We define a function d, our decorator, and in its body is defined a new function r, that
we are going to return. In the body of r a is going to be executed, the decorated function.
Now change the comments for code that actually does something:
Manipulating functions
def sum(a, b):
print a + b
>>> sum(1,2)
3
>>> sum2 = d(sum)
>>> sum2(1,2)
Start of execution of sum
3
Stop of execution of sum
>>> sum = d(sum)
>>> sum(1, 2)
Start of execution of sum
3
Stop of execution of sum
Also we can keep directly in sum the decorated version of sum and now the original
version will not be longer accessible.
Previous way of applying a decorator is the functional form. We have a nicer one:
Syntactic sugar
As of Python 2.4 the @ notation was incorporated for function decorators.