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Date: Sat, 21 Jul 2012 15:35:26 -0600
Subject: Re: A thread import problem
From: Bruce Sherwood <bruce.sherwood@gmail.com>
To: python-list@python.org
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On Sat, Jul 21, 2012 at 2:53 PM, Dave Angel <d@davea.name> wrote:
> On 07/21/2012 04:36 PM, Bruce Sherwood wrote:
>> Thanks much for this clear statement. I hadn't managed to find any
>> documentation on this specific issue.
>>
>> Bruce Sherwood
>>
>> On Sat, Jul 21, 2012 at 10:26 AM, Dave Angel <d@davea.name> wrote:
>>> Two of the things you mustn't do during an import:
>>>
>>> 1) start or end any threads
>>> 2) import something that's already in the chain of pending imports.
>>> (otherwise known as recursive imports, or import loop).  And there's a
>>> special whammy reserved for those who import the script as though it
>>> were a module.
>>>
>>> Like any rule, there are possible exceptions.  But you're much better
>>> off factoring your code better.
>>>
>>> I haven't managed to understand your software description, so i'm not
>>> making a specific suggestion.  But I know others have pointed out that
>>> you should do as little as possible in top-level code of an imported
>>> module.  Make the work happen in a function, and call that function from
>>> the original script, not from inside some import.  An imported module's
>>> top-level code should do nothing more complex than initialize module
>>> constants.
>>>
>>>
>>> --
>>>
>>> DaveA
>>>
>>>
>
> (You top-posted, which makes it harder to figure out who said what.)
>
> For docs on the threading thing, see:
>
> http://docs.python.org/library/threading.html
>
> " ... an import should not have the side effect of spawning a new thread
> and then waiting for that thread in any way..."
>
>
>
>
> --
>
> DaveA
>

Thanks. I had read that as forbidding "waiting for that thread", not
forbidding spawning a new thread. The following sentence says,
"Failing to abide by this restriction can lead to a deadlock if the
spawned thread directly or indirectly attempts to import a module." I
gather that a clearer, more forceful statement might be, "Failing to
abide by this restriction WILL lead to a deadlock if the spawned
thread directly or indirectly attempts to import a module." All of
which implies the behavior I've seen in various experiments, namely
that as long as the spawned thread doesn't do any imports, I haven't
seen any problems with spawning a thread in an imported module. I take
your word for it that this is a no-no, but I don't know why.

Bruce Sherwood