[Python-Dev] New PEP: 319

[Michel Pelletier]

On Tuesday 17 June 2003 04:01, Jack Jansen wrote:
>  but there's one I'd like to elaborate a little on:
>
> Michel Pelletier said:
> > I belive the reverse, synchronize will reduce user error and
> > deadlocking code.  with explicit locks programmers will forget,=20
> > or become confused,
> > with when and how to explicitly lock and unlock.
>
> The problem is if one piece of code has
> =09synchronise a:
> =09=09...
> =09=09synchronise b:
> =09=09=09...
> and somewhere else you have
> =09synchronise b:
> =09=09...
> =09=09synchronise a:
> =09=09=09...

As you pointed out, the programmer must be aware of this when synchronizi=
ng=20
with any mechanism.  Would you prefer the manual say "don't do the above"=
 or=20
"don't do the below":

lock1 =3D thread.allocate_lock()
lock2 =3D thread.allocate_lock()

lock1.acquire()
try:
    lock2.acquire()
    try:
        ...
    finally:
        lock2.release()
finally:
    lock1.release()

# and somewhere else you have

lock2.acquire()
try:
    lock1.acquire()
    try:
        ...
    finally:
        lock1.release()
finally:
    lock2.release()

Maybe I did this wrong, but aren't the two (and Greg's "synchronized clas=
s")=20
all susceptible to this problem and it's not specificly a failure of the=20
'synchronize' keyword?

Thanks for your comments Jack, I'm going to add this to the discussion se=
ction=20
of the PEP.=20

-Michel