[Python-ideas] __before__ and __after__ attributes for functions

[Chris Angelico]

On Thu, Jan 23, 2014 at 7:11 PM, Suresh V. <suresh_vv at yahoo.com> wrote:
> On Thursday 23 January 2014 01:22 PM, Chris Angelico wrote:
>>
>> On Thu, Jan 23, 2014 at 6:20 PM, Suresh V. <suresh_vv at yahoo.com> wrote:
>>>
>>> Can we add these two attributes for every function/method where each is a
>>> list of callables with the same arguments as the function/method itself?
>>>
>>> Pardon me if this has been discussed before. Pointers to past discussions
>>> (if any) appreciated.
>>
>>
>> I'm not exactly sure what you're looking for here. What causes a
>> callable to be added to a function's __before__ list, and/or what will
>> be done with it?
>
>
> These are modifiable attributes, so something can be added/deleted from the
> __before__ or __after__ lists.
>
>
>>
>> If you mean that they'll be called before and after the function
>> itself, that can be more cleanly done with a decorator.
>
>
> Yes. Each item in the list will be called in order immediately before/after
> each invocation of the function. This is kinda like decorators, but more
> flexible and simpler. Scope for abuse may be higher too :-)

def prepostcall(func):
    def wrapper(*args,**kwargs):
        for f in wrapper.before: f(*args,**kwargs)
        ret = func(*args,**kwargs)
        for f in wrapper.after: f(*args,**kwargs)
        return ret
    wrapper.before = []
    wrapper.after = []
    return wrapper

@prepostcall
def foo(x,y,z):
    return x*y+z

foo.before.append(lambda x,y,z: print("Pre-call"))
foo.after.append(lambda x,y,z: print("Post-call"))

Now just deal with the question of whether the after functions should
be called if the wrapped function throws :)

ChrisA