[Python-Dev] With statement

[Duncan Booth]

Greg Ewing <greg@cosc.canterbury.ac.nz> wrote in 
news:200302050337.h153b4v12977@oma.cosc.canterbury.ac.nz:

>>    with lock(my_lock):
>>        # protected code
> 
> But if my_lock is already a lock object, with appropriate enter 
> and leave methods, what would the lock() function do?
> 
> If the answer is
> 
>   def lock(x):
>     return x
> 
> then I'd say that I don't like the idea of a do-nothing
> function whose purpose is only to make the code look
> pretty.
> 

Sorry, I didn't make myself clear. 'lock' here is an adapter class to let 
Threading.Lock be used in a with statement. The definition would be:

class lock:
   def __init__(self, lockable):
       self.lockable = lockable
       lockable.acquire()
   def __exit__(self):
       self.lockable.release()

My point being that the existing proposal with __enter__ and __exit__ may 
be replaced by a simpler proposal with only __exit__ by using a class which 
does the __enter__ functionality in the constructor.

If the proposal was to change the existing Threading.Lock class so that it 
directly supported use in a 'with' statement, then one way would be to add 
__enter__ and __exit__ methods. An alternative which doesn't require 
__enter__ at all would be to add a single 'lock' method that creates a lock 
object as defined above. Then the code might look something like:

    my_lock = Threading.Lock()
    ...
    with my_lock.lock():
        ... protected code ...


-- 
Duncan Booth                                             duncan@rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?