[python-uk] Advice on decorator grammar

[Simon Yarde]

This may well be moot, so thank you for chipping in. All your suggestions  are completely valid and practical.

And thank you Stestagg and a.cavallo for commenting on references; I've tried to show in the examples below how the instance might be used to store config that is accessed by instance-methods, so external access was not an issue for the scenario I was envisaging.

I'm interested in decorator-methods that can be employed in different scenarios; as 'python decorators'; and using a decorator-pattern for dynamic decoration of callables.

The grammar seems to preclude such flexibility, and a certain elegance.

I'll try to set-out a possible flexible design-pattern that shows the same decorator-method employed flexibly, and where it becomes inelegant or unintuitive.

This works:

def f():
    pass

f = A('foo').some_process(f)

This 'could' work, where it not for grammar inconsistency:

@A('foo').some_process
def f():
    pass

The same pattern enables dynamic decoration using different instances of A:

@apply_some_process_from_one_of_these_at_random(
   A('foo'),
   A('bar')
)
def f():
    pass

> You could do this by making decorator_method a classmethod:
> 
> @MyDecorator.decorate_this(foo)


Using a class-method, I would have to name the method I wanted to call and supply initialisation at the same time, and return a configured callable to perform the desired process:

@apply_some_process_from_one_of_these_at_random(
    A.some_process('foo')
    A.some_process('bar')
)

It's attractive not to have to name the process to be called at configuration, and to be able to store configuration in the instance (this works):

@apply_an_arbitrary_process_from(
    A('foo')
)

and it would be elegant/consistent to be able to apply a process using the decorator-syntax if need be (this doesn't work because of the grammar):

@A('foo').some_other_process


On 3 Apr 2013, at 12:43, Stestagg wrote:

> This seems redundant to me, the MyDecorator instance would not be bound to anything, so you'll 'loose' the reference to it, except through the call to decorator_method().  
> 
> You could do this by making decorator_method a classmethod:
> 
> class MyDecorator(object):
> 
>     @classmethod
>     def decorate_this(cls, ...):
>         pass
> 
> allowing you to use it:
> 
> @MyDecorator.decorate_this(foo)
> 
> If your intent is to pass arguments to the MyDecorator instance, just pass them to the decorator method directly.
> 
> Finally, if you're trying to implement singleton like behaviour.  (a registry etc..) then using your example of binding an instance of MyDecorator() to a module-level name is sensible.
> 
> MY_REGISTRY = MyDecorator()
> 
> @MY_REGISTRY.decoate_this()
> def wrapped():
>     ...
> 
> Does your use-case match any of these?
> 
> Thanks
> 
> Steve
> 
> 
> On Wed, Apr 3, 2013 at 12:34 PM, Simon Yarde <simonyarde at me.com> wrote:
> Hi All
> 
> I've not posted to this list before. Hello!
> 
> I have a question about decorators and have failed to devise a search that has thrown up any history of discussion on this particular matter.
> 
> Does the following seem like something that 'should' work? Or is anyone aware of a source of documentation that explains historically why the following syntax might not be allowed?
> 
> I hope this sort of conundrum/discussion-point is appropriate to this forum; I'm not on python-dev and this is obviously not a bug.
> 
> So..
> 
> Decorator grammar is this:
> 
> decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE
> 
> The grammar prevents this:
> 
> >>> class MyDecorator:
> ...     def decorator_method():
> ...             pass
> ...
> >>> @MyDecorator().decorator_method()
>   File "<stdin>", line 1
>     @MyDecorator().decorator_method()
>                   ^
> SyntaxError: invalid syntax
> 
> But is possible to achieve the desired effect by assigning the class instance to variable:
> 
> >>> mydecorator = MyDecorator()
> ... @mydecorator.decorator_method
> ... def f():
> 
> 
> My initial thoughts were that the syntax provided a neat way to provide a configurable decorator class instance with a number of alternative decorator-function generating methods, rather than just the usual __call__.
> 
> S
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Simon Yarde

07525 063 134
simonyarde at me.com