how to write thread-safe module ? and pytz

[Daniel Dittmar]

nicolas_riesch wrote:
> Does someone know if the module pytz
> (http://sourceforge.net/projects/pytz/) is thread-safe ?
> I have not seen it explicitely stated, and just wanted to be sure, as I
> want to use it.
> 
> That's because in the file pytz/tzinfo.py, I see global variables
> _timedelta_cache, _datetime_cache, _ttinfo_cache, which are
> dictionnaries and are used as cache.
> I always thought that you must protect shared data with locks when
> multithreading, but I don't see any lock anywhere in pytz.

Dictionaries (and probably most other Python types that are implemented 
in C) are inherently thread safe.

This applies only  to the individual methods of dictionaries. The 
following code would still require a lock:
if mydict.has_key (keyval):
     variable = mydict [keyval]
because a second thread could delete the entry between the calls to 
has_key and __getvalue__.

mydict [keyval] = mydict.get (keyval, 0) + 1
is also an candidate for problems.

> However, pytz seems to work well with multiple threads creating various
> timezone objects at the same time.

'Seems to work' is never a good argument with regard to threads. 
Especially if you're testing on a single CPU machine.

Daniel