Annotations AKA Decorators in Python

Today I came across some Python code where the one method had what looked like a Java annotation above it:

@property
def get_name():
    return self.name

This is actually what is called a decorator in Python. It is simply a function that works on another function (a higher order function). The @method_name is syntactic sugar for wrapping the method it is above.

For example say I wanted to print out hello before every method call I would first define a function as follows:

def say_hello(fun):
    def wrapper():
        print("Hello")
        fun()
    return wrapper

I would then use it as:

@say_hello
def bar():
    print("Bar")

bar()

It is important you return a function as in the above. If you by mistake do the following instead:

def say_hello(fun):
    print("Hello")
    fun()

@say_hello
def bar():
    print("Bar")

bar()

You will get the following output and error (deocrator.py is the name of the file where I had this example code in):

Hello
Bar
Traceback (most recent call last):
  File "decorator.py", line 9, in <module>
    bar()
TypeError: 'NoneType' object is not callable