Re: dictionaries and threads

Discussion in 'Python' started by Tim Peters, Jun 2, 2005.

  1. Tim Peters

    Tim Peters Guest

    [Gary Robinson]
    > I know the Global Interpreter Lock ensures that only one python thread
    > has access to the interpreter at a time, which prevents a lot of
    > situations where one thread might step on another's toes.


    Not really. The CPython implementation's C code relies on the GIL in
    many ways to provide mutual exclusion, but there's no such effect at
    the Python level: operations from multiple Python threads can get
    interleaved in any order consistent with sequential execution within
    each thread. The guarantee that the builtin datatypes provide "not to
    go insane" in the absence of application synchronization is really a
    separate matter, _aided_ in CPython by the GIL, but not guaranteed by
    the GIL alone. There's still an enormous amount of delicate fiddling
    in Python's C code to keep internals sane in the absence of
    application locking.

    > But I'd like to ask about a specific situation just to be sure I
    > understand things relative to some code I'm writing.
    >
    > I've got a dictionary which is accessed by several threads at the same
    > time (that is, to the extent that the GIL allows). The thing is,
    > however, no two threads will ever be accessing the same dictionary
    > items at the same time. In fact the thread's ID from thread.get_ident()
    > is the key to the dictionary; a thread only modifies items
    > corresponding to its own thread ID. A thread will be adding an item
    > with its ID when it's created, and deleting it before it exits, and
    > modifying the item's value in the meantime.
    >
    > As far as I can tell, if the Python bytecodes that cause dictionary
    > modifications are atomic, then there should be no problem. But I don't
    > know that they are because I haven't looked at the bytecodes.


    It's unclear what kind of problem you're concerned about. The kind of
    dict you described is widely used in Python apps; for example, it's at
    the heart of the implementation of threading.currentThread() in the
    standard library. That dicts promise not to go insane in the absence
    of locking isn't a GIL issue, even if multiple threads do modify d[k]
    simultaneously (in the absence of app locking, all such threads will
    eventually change d[k], but in an undefined order; if no two threads
    can muck with the same k, there's no problem at all).

    > Any feedback on this would be appreciated. For various reasons, we're
    > still using Python 2.3 for the time being.


    Best practice in 2.3 is to subclass threading.Thread for all threads
    you use. Then instead of mucking with a dict, you can just
    set/retrieve an attribute on the current Thread object. You can get
    the currently active Thread at any time via calling
    threading.currentThread().
     
    Tim Peters, Jun 2, 2005
    #1
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  2. Tim Peters

    Bloke Guest

    I've been trying to find a Python tutorial on threading - does anyone
    have a reference?

    Rob
     
    Bloke, Jun 2, 2005
    #2
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  3. Tim Peters

    John Abel Guest

    John Abel, Jun 2, 2005
    #3
  4. Tim Peters

    Bloke Guest

    thanks. Looks good.
     
    Bloke, Jun 2, 2005
    #4
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