Python Cookbook

Storing Per-Thread Information

Credit: John E. Barham

Problem

You need to allocate storage to each thread for objects that only that thread can use.

Solution

Thread-specific storage is a useful pattern, and Python does not support it directly. A simple dictionary, protected by a lock, makes it pretty easy to program. For once, it’s slightly more general, and not significantly harder, to program to the lower-level thread module, rather than to the more common, higher-level threading module that Python also offers on top of it:

try:
    import thread
except:
    """ We're running on a single-threaded OS (or the Python interpreter has
    not been compiled to support threads), so return a standard dictionary. """
    _tss = {}
    def get_thread_storage(  ):
        return _tss
else:
    """ We do have threads; so, to work: """
    _tss = {}
    _tss_lock = thread.allocate_lock(  )
    def get_thread_storage(  ):
        """ Return a thread-specific storage dictionary. """
        thread_id = thread.get_ident(  ) # Identify the calling thread
        tss = _tss.get(thread_id)
        if tss is None: # First time being called by this thread
            try: # Entering critical section
                _tss_lock.acquire(  )
                _tss[thread_id] = tss = {} # Create thread-specific dictionary
            finally:
                _tss_lock.release(  )
        return tss

Discussion

The get_thread_storage function in this recipe returns a thread-specific storage dictionary. It is a generalization of the get_transaction function from ZODB, the object database underlying Zope. The returned dictionary can be used to store data that is private to ...