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FAQ
Hi,if I have a class A that contains a boolean variable named x, is it safe
to read and change it from different threads without using locks?
Is it guaranteed that A.x will be always True or False, and not any other
weird value that that causes it to be inconsistent (assuming I only set it
to True or False) ?

I have a = A()
first thread does:
if a.x is True :
pass

2nd thread does:
a.x = False

is it safe?

and what if x was a dict ? especially if the only values that are set in the
dictionary are simple: booleans, integers, floats, strings
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  • Steve at Sep 30, 2009 at 8:44 am ⇧
    Hi,
    On 09/30/2009 01:53 PM, Charlie Dickens wrote:
    Hi,
    if I have a class A that contains a boolean variable named x, is it safe
    to read and change it from different threads without using locks?
    Is it guaranteed that A.x will be always True or False, and not any
    other weird value that that causes it to be inconsistent (assuming I
    only set it to True or False) ?
    The guarantee for A.x being only True or False, is independent of whether you
    use locks or not. It depends entirely on code that assigns to A.x.
    I have a = A()
    first thread does:
    if a.x is True :
    pass

    2nd thread does:
    a.x = False

    is it safe?
    what if you have code like this:

    Thread 1
    if a.x is True:
    doSomething()

    Thread 2
    a.x == False

    ..and thread 2 executes *after* thread 1's 'if' condition but *before*
    doSomething(). If that behavior is acceptable in your application, you possibly
    don't even need a.x.
    and what if x was a dict ? especially if the only values that are set in
    the dictionary are simple: booleans, integers, floats, strings
    Same logic applies.

    hth,
    cheers,
    - steve

    --
    random non tech spiel: http://lonetwin.blogspot.com/
    tech randomness: http://lonehacks.blogspot.com/
    what i'm stumbling into: http://lonetwin.stumbleupon.com/
  • Charlie Dickens at Sep 30, 2009 at 9:14 am ⇧
    What about dictionaries? Reading values, adding new ones and the most
    important: changing an existing value - can it corrupt the state of the
    dictionary or that it is guaranteed that if I try to read the value of this
    key, I can only get the old one or the new one, but not something weird
    instead (because a different thread changed the value while this thread was
    trying to read it).
    About your remark that I don't need to check if a.x is True, I do need, and
    indeed it is acceptable that doSomething() will run, because this logic
    resides inside a loop, and it is only important for me that it will stop
    entering the loop sometime (I don't care if there are n iterations or n+1 or
    even n+m)

    Thanks
    On Wed, Sep 30, 2009 at 10:44 AM, steve wrote:

    Hi,
    On 09/30/2009 01:53 PM, Charlie Dickens wrote:

    Hi,
    if I have a class A that contains a boolean variable named x, is it safe
    to read and change it from different threads without using locks?
    Is it guaranteed that A.x will be always True or False, and not any
    other weird value that that causes it to be inconsistent (assuming I
    only set it to True or False) ?

    The guarantee for A.x being only True or False, is independent of whether
    you use locks or not. It depends entirely on code that assigns to A.x.

    I have a = A()
    first thread does:
    if a.x is True :
    pass

    2nd thread does:
    a.x = False

    is it safe?
    what if you have code like this:

    Thread 1
    if a.x is True:
    doSomething()

    Thread 2
    a.x == False

    ..and thread 2 executes *after* thread 1's 'if' condition but *before*
    doSomething(). If that behavior is acceptable in your application, you
    possibly don't even need a.x.

    and what if x was a dict ? especially if the only values that are set in
    the dictionary are simple: booleans, integers, floats, strings

    Same logic applies.
    hth,
    cheers,
    - steve

    --
    random non tech spiel: http://lonetwin.blogspot.com/
    tech randomness: http://lonehacks.blogspot.com/
    what i'm stumbling into: http://lonetwin.stumbleupon.com/
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  • Gabriel Genellina at Oct 6, 2009 at 2:08 am ⇧
    En Wed, 30 Sep 2009 06:14:50 -0300, Charlie Dickens
    <cdickens123 at gmail.com> escribi?:
    What about dictionaries? Reading values, adding new ones and the most
    important: changing an existing value - can it corrupt the state of the
    dictionary or that it is guaranteed that if I try to read the value of
    this
    key, I can only get the old one or the new one, but not something weird
    instead (because a different thread changed the value while this thread
    was
    trying to read it).
    See http://effbot.org/zone/thread-synchronization.htm

    --
    Gabriel Genellina

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postedSep 30, '09 at 8:23a
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