Using arguments in a decorator

[Steven D'Aprano]

On Fri, 20 Apr 2012 09:10:15 -0700, Jon Clements wrote:

>> But I don't know how. I know that I can see the default arguments of
>> the original function using func.__defaults__, but without knowing the
>> number and names of func's positional arguments (which I don't know how
>> to find out) this doesn't help me. Any suggestions?
> 
> Possibly take a look at functools.lru_cache (which is Python 3.2+), and
> use the code from that (at it's part of the stdlib, someone must have
> done design and testing on it!).

With respect Jon, did you read the Original Poster's question closely? 
Using a LRU cache doesn't even come close to fixing his problem, which 
occurs *before* you do the lookup in the cache.

Rotwang's problem is that if you have a function with default arguments:

def func(spam=42):
    return result_of_time_consuming_calculation()

then these three function calls are identical and should (but don't) 
share a single cache entry:

func()
func(42)
func(spam=42)

The OP would like all three to share a single cache entry without needing 
two redundant calculations, which take a long time.

The problem is that the three calls give three different patterns of args 
and kwargs:

(), {}
(42,) {}
(), {'spam': 42}

hence three different cache entries, two of which are unnecessary.



-- 
Steven