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Re: [pygame] @

To: pygame-users@xxxxxxxx
Subject: Re: [pygame] @
From: Yanom Mobis <yanom@xxxxxxxxxxxxxx>
Date: Thu, 1 Jan 2009 11:56:16 -0800 (PST)
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oh.


--- 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, 6:40 PM

So doing the same conversion we start with

def a(b): b()

@a
def eggs(): print 'spam'

This becomes:

def a(b): b()

def eggs(): print 'spam'
eggs = a(eggs)

If you look your a() function doesn't return anything so eggs is set  
to the default return value which is always None.

--Noah

On Dec 31, 2008, at 7:37 PM, Yanom Mobis wrote:

> I think I've got it now. But why does this happen?
>
>>>> def a(b):
> ...   b()
> ...
>>>> @a
> ... def eggs():
> ...   print "spam"
> ...
> spam
>>>> eggs()
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> TypeError: 'NoneType' object is not callable
>>>>
>
>
>
>
>
>
> --- On Wed, 12/31/08, Lenard Lindstrom <len-l@xxxxxxxxx> wrote:
>
> From: Lenard Lindstrom <len-l@xxxxxxxxx>
> Subject: Re: [pygame] @
> To: pygame-users@xxxxxxxx
> Date: Wednesday, December 31, 2008, 4:05 PM
>
> 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>
>
>
>
>
>
>

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