Re: [google-appengine] Re: Thread-safe Python Tips
On Wed, Jul 6, 2011 at 8:00 AM, Pol i...@pol-online.net wrote: On Jul 1, 10:10 am, Ikai Lan (Google) ika...@google.com wrote: It's possible for two operations to update townCache concurrently, but in your case it looks like it doesn't really matter. If TownModel is somehow updated between reads, it's theoretically possible for you to have an older TownModel in the local cache, but if you're going to store something in the cache with no expiration, it sounds like you don't care about this case anyway. Are collections thread-safe in Python? Otherwise, townCache[id] = ... called at the same time on multiple threads would likely corrupt something. http://en.wikipedia.org/wiki/Global_Interpreter_Lock -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
[google-appengine] Re: Thread-safe Python Tips
On Jul 1, 10:10 am, Ikai Lan (Google) ika...@google.com wrote: It's possible for two operations to update townCache concurrently, but in your case it looks like it doesn't really matter. If TownModel is somehow updated between reads, it's theoretically possible for you to have an older TownModel in the local cache, but if you're going to store something in the cache with no expiration, it sounds like you don't care about this case anyway. Are collections thread-safe in Python? Otherwise, townCache[id] = ... called at the same time on multiple threads would likely corrupt something. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
Re: [google-appengine] Re: Thread-safe Python Tips
On Tue, Jul 5, 2011 at 8:47 AM, Joshua Smith joshuaesm...@charter.netwrote:
Thanks for that clarification. I'm sure there's a reason for this
asymmetry (must declare globals to write them, but not to read them), but
it's really weird.
Since Python doesn't require variable declaration at all, there's no way for
it to know if foo = 3 is intended to create a local or a global without an
explicit declaration. If it assumed a local unless a global already existed,
creating a global in your module could unexpectedly change the behavior of a
function that happened to use the same name for a local variable.
-Nick Johnson
On Jul 1, 2011, at 6:12 PM, Geoffrey Spear wrote:
On Jun 29, 1:40 pm, Joshua Smith joshuaesm...@charter.net wrote:
I have this code in one of my apps:
townCache = {}
def getTown(id):
if not id in townCache:
townCache[id] = TownModel.get_by_id(id)
return townCache[id]
Is this thread safe? I think it is, because the worst that happens is
the assignment happens redundantly with the same data.
Random other question: Why don't I have to say global townCache at the
top of that function?
You can't *assign* to a global variable in another scope without the
global keyword; however, townCache is the global name here, not
townCache[id].
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Re: [google-appengine] Re: Thread-safe Python Tips
Thanks for that clarification. I'm sure there's a reason for this asymmetry
(must declare globals to write them, but not to read them), but it's really
weird.
On Jul 1, 2011, at 6:12 PM, Geoffrey Spear wrote:
On Jun 29, 1:40 pm, Joshua Smith joshuaesm...@charter.net wrote:
I have this code in one of my apps:
townCache = {}
def getTown(id):
if not id in townCache:
townCache[id] = TownModel.get_by_id(id)
return townCache[id]
Is this thread safe? I think it is, because the worst that happens is the
assignment happens redundantly with the same data.
Random other question: Why don't I have to say global townCache at the top
of that function?
You can't *assign* to a global variable in another scope without the
global keyword; however, townCache is the global name here, not
townCache[id].
--
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[google-appengine] Re: Thread-safe Python Tips
On Jun 29, 1:40 pm, Joshua Smith joshuaesm...@charter.net wrote:
I have this code in one of my apps:
townCache = {}
def getTown(id):
if not id in townCache:
townCache[id] = TownModel.get_by_id(id)
return townCache[id]
Is this thread safe? I think it is, because the worst that happens is the
assignment happens redundantly with the same data.
Random other question: Why don't I have to say global townCache at the top
of that function?
You can't *assign* to a global variable in another scope without the
global keyword; however, townCache is the global name here, not
townCache[id].
--
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[google-appengine] Re: Thread-safe Python Tips
We are not going to have to go into that much detail for our apps thought are we. Will we be able to take advantage of the multi-threading by simply creating multiple request handlers. I imagined I would be changing a few lines of code in my main function an that's about it. On Jun 29, 9:43 pm, Joshua Smith joshuaesm...@charter.net wrote: I would assume they are the same as the basic principles of thread safety in any language: - Don't rely on global state, because multiple of your functions might be running simultaneously This usually isn't very hard to achieve - just pass parameters instead of modifying globals. The places where it can get tricky are where you really *want* to use global state, such as for an in-memory cache. Usually the language provides some primitives to ensure that only one thread at a time is updating the cache. It appears that python gives you thread-safety for a lot of cases: http://effbot.org/zone/thread-synchronization.htm On Jun 29, 2011, at 1:20 AM, Greg wrote: Hi - Could anyone familiar with threads explain the basic principals of python thread-safety? Cheers! Greg. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group athttp://groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
Re: [google-appengine] Re: Thread-safe Python Tips
I don't believe you'll need to do anything when we ship concurrent requests for Python, as long as you're not using global mutable state anywhere. It's unlikely your threads will ever need to talk to each other. Ikai Lan Developer Programs Engineer, Google App Engine Blog: http://googleappengine.blogspot.com Twitter: http://twitter.com/app_engine Reddit: http://www.reddit.com/r/appengine On Thu, Jun 30, 2011 at 4:57 AM, jay kyburz@gmail.com wrote: We are not going to have to go into that much detail for our apps thought are we. Will we be able to take advantage of the multi-threading by simply creating multiple request handlers. I imagined I would be changing a few lines of code in my main function an that's about it. On Jun 29, 9:43 pm, Joshua Smith joshuaesm...@charter.net wrote: I would assume they are the same as the basic principles of thread safety in any language: - Don't rely on global state, because multiple of your functions might be running simultaneously This usually isn't very hard to achieve - just pass parameters instead of modifying globals. The places where it can get tricky are where you really *want* to use global state, such as for an in-memory cache. Usually the language provides some primitives to ensure that only one thread at a time is updating the cache. It appears that python gives you thread-safety for a lot of cases: http://effbot.org/zone/thread-synchronization.htm On Jun 29, 2011, at 1:20 AM, Greg wrote: Hi - Could anyone familiar with threads explain the basic principals of python thread-safety? Cheers! Greg. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com . To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group athttp:// groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
