atomic increment

[Alexandru Mosoi]

how can i do an atomic read+increment? something like

with lock:
old = atomic_int
atomic_int += 1

but in one operation

 

[Diez B. Roggisch]

how can i do an atomic read+increment? something like

with lock:
old = atomic_int
atomic_int += 1

but in one operation

As above - the lock (under the assumption that it is actually a
threading.Lock) will ensure that.

Diez

 

[Frank Millman]

As above - the lock (under the assumption that it is actually a
threading.Lock) will ensure that.

Diez

Just out of interest, would the following, without a lock, be safe?

old, atomic_int = atomic_int, atomic_int+1

Frank Millman

 

[Paul Rubin]

Just out of interest, would the following, without a lock, be safe?
old, atomic_int = atomic_int, atomic_int+1

No I don't think so. But I'm told that in CPython, you can say

counter = iter(xrange(10000000)) # some number that exceeds what you'll use
...

atomic_int = counter.next()

and the GIL keeps it safe. When in doubt, use a lock or communicate
with other threads through Queues.

 

[Fredrik Lundh]

Just out of interest, would the following, without a lock, be safe?

old, atomic_int = atomic_int, atomic_int+1

nope.

there's some information here (make sure you read the comments):

http://effbot.org/pyfaq/what-kinds-of-global-value-mutation-are-thread-safe.htm

and some additional discussion here:

http://mail.python.org/pipermail/python-dev/2006-November/thread.html#69981

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