Decorators in Python

A decorator is a pretty simple idea. It is a function that modifies another function.

Or in the case of @classmethod it is a function that modifies a whole class.

It is analogous to taking any value, transforming it in some way and returning a new value.

def square(i):
    return i**2
x = square(x)

No big deal. We have just changed the value of x from some integer to the square of that integer. This is basic programming in Python.

But what if the value we are taking is not an integer but a function:

def logger(f):

    def with_log(*args, **kwargs):
        print "Arguments: %s, %s" % (args, kwargs)
        return f(*args, **kwargs)

    return with_log

f = logger(f)

In this case, we are taking some function f and printing out some logging information before returning the function (thanks to Simeon Franklin for this example).

None of the work is done until f is actually called. When it is, the arguments used are passed to the with_log function defined in logger. This prints out some logging information before returning the original function.

This, by the way, is an example of a closure (from which the clojure programming language gets its name), where theiinner with_log function retains the value of the function f, which is define in the outer logger function, i.e. the inner function encloses the outer function.

Using the decorator syntax and assuming that the logger function is already defined, instead of writing:

def f():
    <define f here>

f = logger(f)

we can write:

def f():
    <define f here>

It is a simple difference, but having the decorator directly above the function definition aids readability. It is just syntactic sugar.

In the case of a class method:

def by_id(cls, uid):
   return User.get_by_id(uid)

is equivalent to:

def by_id(cls, uid):
    return User.get_by_id(uid)

by_id = classmethod(by_id)

It is an easy way to announce that a method is intended not to act on a single instance of a class, but instead on the whole class.

It is nothing particularly clever. It is just telling the reader, “watch out, this is a class method”.

That’s it.

If you want more information, Bruce Eckel has written a good Introduction to Python Decorators.