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Re: [pygame] @

A concrete example helps. Unfortunately, knowing how a decorator works does nothing to explain how it is being used any more than know how to declare a function tells you what all functions do. Anyway, the key thing to remember is a function declaration in Python is an executable statement, a function instance is created when execution reaches the declaration, and that instance is assigned to the variable of the same name. So: 

>>> def fn(x):
   return x

>>> fn
<function fn at 0x00C74970>
>>> fn(1)
1
>>> fn = 'this is not a function'
>>> fn
'this is not a function'
when executed, creates an new function instance and assigns it to identifier fn. However, a decorator is just a function itself. So, 

>>> def decorator(f):
   print f
   return f

>>> @decorator
def fn(x):
   return x

<function fn at 0x00C64870>
>>> fn
<function fn at 0x00C64870>
An instance of function fn is created, then function decorator is called with that instance as its only argument. decorator then returns the function instance, which is assigned to identifier fn. So a decorator is called when a function declaration is executed, not later when the function is called.
In the pyglet example "window.event" is a method that accepts a function as an argument. In this case 'do_draw' is likely a callback, a function that will be called whenever the window receives a draw event. This alternate code probably would also work: 

def on_draw():
   ....
window.event(on_draw)   # return value ignored.

It just saves having to repeat the on_draw function name.

Lenard


Yanom Mobis wrote:

oh. They showed up in some basic example code for pyglet:

import pyglet
window = pyglet.window.Window()
label = pyglet.text.Label('Hello, world',
                          font_name='Times New Roman',
                          font_size=36,
                          x=window..width//2, y=window.height//2,
                          anchor_x='center', anchor_y='center')
@window.event
def on_draw():
    window.clear()
    label.draw()
pyglet.app.run()



--- On Wed, 12/31/08, Noah Kantrowitz <noah@xxxxxxxxxxxxxx> wrote:

From: Noah Kantrowitz <noah@xxxxxxxxxxxxxx>
Subject: Re: [pygame] @
To: pygame-users@xxxxxxxx
Date: Wednesday, December 31, 2008, 1:59 PM
No, as I said, decorators are a rather advanced topic. Until you are more comfortable with the whole compiler process, I would just try writing a few small ones with various prints to see what happens. 

--Noah

On Dec 31, 2008, at 2:57 PM, Yanom Mobis wrote:


class compile?
Anyway, does it effectively work that way?



--- On Wed, 12/31/08, Noah Kantrowitz <noah@xxxxxxxxxxxxxx> wrote:

From: Noah Kantrowitz <noah@xxxxxxxxxxxxxx>
Subject: Re: [pygame] @
To: pygame-users@xxxxxxxx
Date: Wednesday, December 31, 2008, 1:41 PM

No, i has nothing to do with runtime. Decorators are evaluated during
class compile.

--Noah

On Dec 31, 2008, at 12:05 PM, Yanom Mobis wrote:


Ohhhh!  I get it now! It's used to insure that a specific function
is always called before another. Thanks for clearing it up for me..
--- On Wed, 12/31/08, Michael Phipps <michael.phipps@xxxxxxxxxxx>
wrote:

From: Michael Phipps <michael.phipps@xxxxxxxxxxx>
Subject: Re: [pygame] @
To: pygame-users@xxxxxxxx
Date: Wednesday, December 31, 2008, 4:37 PM

Yanom -

A decorator is a method that takes another method as a parameter so
that it can do something.. It is usually used for aspect oriented
programming.

For example:

def logThisMethodCall(methodCall)
      # Do some logging here

@logThisMethodCall
def myMethod(a,b,c)
      # do Somthing in here

Now, whenever you call "myMethod", logThisMethodCall gets called
first, with the invocation of myMethod passed into it. You can use
it for logging, security (i.e. does this person have permission to
be calling this), etc.

Michael




-----Original Message-----
From: "Yanom Mobis" [yanom@xxxxxxxxxxxxxx]
Date: 12/31/2008 11:19
To: pygame-users@xxxxxxxx
Subject: Re: [pygame] @

so when you do this:

@foo
def bar(): pass

you assume that a function foo() already exists..

and it creates something like this:

def foo():
      def bar(): pass
      pass

?
I'm sorry, I just got confused.




- On Wed, 12/31/08, Noah Kantrowitz <noah@xxxxxxxxxxxxxx> wrote:
From: Noah Kantrowitz <noah@xxxxxxxxxxxxxx>
Subject: Re: [pygame] @
To: pygame-users@xxxxxxxx
Date: Wednesday, December 31, 2008, 3:01 AM

decorator. The short version is that this

@foo
def bar(): pass

is the same as this

def bar(): pass
bar = foo(bar)

The long version is "look it up because it gets very complicated and
voodoo-ish"

--Noah

On Dec 30, 2008, at 9:55 PM, Yanom Mobis wrote:


I was reading some Python code examples, and i found the @ symbol.
What

exactly does this operator do?


--
Lenard Lindstrom
<len-l@xxxxxxxxx>