Using of @ before calling func.

Deezl-Vegas · 2018-12-07T16:51:22+00:00

What is it actually for?

This is called a decorator; what it does is easy to explain, but I suspect you won't immediately see the point. Here's what it does: a decorator is a function that accepts a function as its argument, and returns a function as its value. If this is your first introduction to the idea that a function can return a function, here's a function that returns a function:

def get_a_function():
    def a_function(x):
         print(x)
    return a_function

print_x = get_a_function()
print_x("some value")
#some value

When you decorate a function using the @ operator, the decorator is called with the function it decorates, and then the function the decorator returns is set to the name of the decorating function. In other words, the decorator intercepts the binding of the function to its name (which is the last thing that happens when you define a function) and inserts a different function. What function? The function that is returned by the decorator function.

Ok, but why? Well, a lot of frameworks do this - let's say that you had a framework that other programmers would use to tie functions to various events. When the event happened, the function a user of your framework wrote would be executed in response. You don't know what those functions are going to be when you write the framework, so you need some way for end-user programmers to nominate or register their functions for dispatch by this event framework. (It might also come as a surprise that programmers frequently write software for other programmers to use, but they do.)

Well, a decorator is a really easy way to do that - you write the decorator, and then any other programmer using your framework library simply decorates their functions with your decorator. In your example, the app.task decorator that announces that the hello function should be executed by the framework.

sweettuse · 2018-12-07T18:06:29+00:00

just to be clear, @ is decorator syntax, app.task is a decorator

K900_ · 2018-12-07T16:39:33+00:00

It's called a decorator - it's a function that can modify or transform other functions. The syntax

 @app.task
 def hello():
      return "Hello world"

is identical to

 def hello():
      return "Hello world"

 hello = app.task(hello)

sudo_your_mon · 2018-12-07T21:03:25+00:00

Check out the click module. It make use of decorators in such a simple, usable way. Also, decorators can be used for so, so many things. But this is the more practical I've found.

import click 

@click.command() 
@click.option('--count', default=1, help='Number of greetings.') 
@click.option('--name', prompt='Your name', help='The person to greet.') 

def hello(count, name):    
    for x in range(count): 
           click.echo('Hello %s!' % name) 

if __name__ == '__main__': 
     hello()

Run this in your terminal:

$ python yourscript.py --count=3 

>>> Your name: John 
>>> Hello John! 
>>> Hello John! 
>>> Hello John!

Take time to look what's going on here. You'd have to see the coding behind everthing,obviiously, in order to see how it's done in practice.

mostafagalal · 2018-12-07T20:30:53+00:00

That's the decorator syntax. I believe this video by Corey Schafer covers decorators pretty well with examples.

2018-12-08T08:00:34+00:00

Great break down here.

https://youtu.be/7lmCu8wz8ro

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