Where to place imports

Jean-Paul Calderone exarkun at divmod.com
Fri Jan 23 23:26:22 CET 2009


On Fri, 23 Jan 2009 22:49:34 +0100, Christian Heimes <lists at cheimes.de> wrote:
>Jean-Paul Calderone schrieb:
>>> BTW, if you ever find you are starting to write multi-threaded
>>> applications
>>> then you'll really regret it if you reuse code which does imports from
>>> inside functions. If two or more threads try to import a module
>>> simultaneously then one of them might find it that not everything is
>>> defined in the module exists when it tries to use it.
>>
>> What makes you say this?  There is an import lock (beyond the GIL) which
>> should prevent a thread from ever seeing a partially initialized module.
>
>Partially initialized modules aren't the issue here. One can easily
>create a dead lock situation with a mix of threads and imports.
>
>Example:
>
>* module 'mod_a' is imported [import lock is acquired]
>* module 'mod_a' spawns a new thread
>* the new thread tries to import another module 'mod_b'
>
>Python dead locks here and there is no chance to resolve the lock. The
>import of 'mod_b' is waiting for the first import. The first import
>hasn't finished because it's still executing the body of 'mod_a'.
>
>In order to avoid the situation you *must* never import a module in a
>thread other than the main thread. You should never start a thread in
>the body of a submodule, too. A large application should import all its
>modules first and then create its threads.
>
>If you really need to import a module from a sub thread or a method that
>might be called from a sub thread then you should use the C function
>PyImport_ImportModuleNoBlock() or use sys.modules and imp.lock_held().
>

Of course this is all true and good to know (and I did already ;), but it
doesn't sound like the case being described in the message I was responding
to.

Jean-Paul



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