Function decorator that caches function results

[Ron Adam]

Fredrik Lundh wrote:
> Ron Adam wrote:
> 
> 
>>In effect, 'cache' and 'fn' are replaced by the objects they reference
>>before the cached_result function is returned.
> 
> 
> not really; accesses to "free variables" always go via special cell objects
> (rather than direct object references), and the compiler uses special byte
> codes for all accesses to such objects.
> 
> this snippet illustrates the differences:
> 
> def func1():
>     return a
> 
> def func2(a):
>     return a
> 
> def wrapper(a):
>     def func3():
>         return a
>     return func3
> 
> def decode(func):
>     import dis
>     print func.__name__ + ":"
>     dis.dis(func)
>     print "co_freevars =", func.func_code.co_freevars
>     print "func_closure =", func.func_closure
>     print
> 
> decode(func1)
> decode(func2)
> decode(wrapper(10))
> 
> if you run this on a recent version of Python, you get something like:
> 
>     func1:
>       2           0 LOAD_GLOBAL              0 (a)
>                   3 RETURN_VALUE
>     co_freevars = ()
>     func_closure = None
> 
>     func2:
>       5           0 LOAD_FAST                0 (a)
>                   3 RETURN_VALUE
>     co_freevars = ()
>     func_closure = None
> 
>     func3:
>       9           0 LOAD_DEREF               0 (a)
>                   3 RETURN_VALUE
>     co_freevars = ('a',)
>     func_closure = (<cell at 0x0092C970: int object at 0x008A537C>,)
> 
> note the use of LOAD_DEREF for the variable access.
> 
> Other opcodes include STORE_DEREF (that updates a local variable
> through a cell, used inside the "defining" scope), and MAKE_CLOSURE
> (which sets up the function object for functions that actually uses
> nested scopes; this is where the func_closure attribute is initialized).
> 
> (now, if I weren't limited to plain text, I could have drawn a nice little
> diagram for you, but that's an entirely different thread...)
> 
> </F>

Thanks Fred,  That's something useful to know when working with 
decorators I think.  And it should answer Stevens question too.

Abstract terminology is great once you already know the things and 
concepts it refers to.  But if you don't, finding where the abstract 
connects to reality is sometimes not easy.  There's nothing better than 
a good example to illistrate the concept.  ;-)

Cheers,
    Ron