[Python-ideas] 'where' statement in Python?

[Nick Coghlan]

On Fri, Jul 23, 2010 at 6:49 AM, George Sakkis <george.sakkis at gmail.com> wrote:
> On Thu, Jul 22, 2010 at 10:21 PM, geremy condra <debatem1 at gmail.com> wrote:
>
>> I would also argue that the more valid comparison would be nested functions
>> or classes- both perfectly pretty constructs on their own- which would cause
>> me to gnaw on otherwise unoffending office furniture if I encountered them
>> nested 3 deep.
>
> I guess you're not much fond of decorators then; it's common to define
> them exactly as 3-level deep nested functions:
>
> def decofactory(deco_arg):
>     def decorator(func):
>        def wrapper(*args, **kwargs):
>            if deco_arg:
>               return func(*args, **kwargs)
>        return wrapper
>    return decorator
>
> @decofactory(n)
> def func(x, y):
>    ...

Actually, I think that's the main reason why parameterised decorators
can be such a pain to understand - keeping the 3 scopes straight in
your head is genuinely difficult. There's a reason the recently added
contextlib.ContextDecorator is implemented as a class with a __call__
method rather than using nested functions.

A given clause would let you reorder this code, and I think doing so
is genuinely clearer:

def decofactory(deco_arg):
    return decorator given:
        def decorator(func):
            if not deco_arg:
                return func
            return wrapper given:
                @wraps(func)
                def wrapper(*args, **kwargs):
                    return func(*args, **kwargs)

Reversing the order of some aspects of the execution allows the text
flow to match the way you would describe the operation: we have a
decorator factory that returns a decorator that returns the function
unmodified if deco_arg evaluates to False and returns a wrapper around
the decorated function otherwise.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia