.. _threaded-imports:

Importing in threaded code

While the import machinery is thread-safe, there are two key restrictions on

threaded imports due to inherent limitations in the way that thread-safety is

provided:

* Firstly, other than in the main module, an import should not have the

side effect of spawning a new thread and then waiting for that thread in

any way. Failing to abide by this restriction can lead to a deadlock if

the spawned thread directly or indirectly attempts to import a module.

* Secondly, all import attempts must be completed before the interpreter

starts shutting itself down. This can be most easily achieved by only

performing imports from non-daemon threads created through the threading

module. Daemon threads and threads created directly with the thread

module will require some other form of synchronization to ensure they do

not attempt imports after system shutdown has commenced. Failure to

abide by this restriction will lead to intermittent exceptions and

crashes during interpreter shutdown (as the late imports attempt to

access machinery which is no longer in a valid state).