Doubley imported module caused devastating bug

Zac Burns zac256 at
Thu Sep 24 22:52:20 CEST 2009

On Thu, Sep 24, 2009 at 1:38 PM, Carl Banks <pavlovevidence at> wrote:
> On Sep 24, 10:26 am, Zac Burns <zac... at> wrote:
>> Currently it is possible to import a file of one path to more than one
>> 'instance' of a module. One notable example is "import __init__" from
>> a package. See
>> This recently caused a devastating bug in some of my code. What I have
>> is support for the Perforce global options as a context for a perforce
>> module.
>> This way one can call functions that call many perforce command and
>> have them execute on a different client for example.
>> So, in module A and module B both imported the Perforce module, but
>> they turned out not to be the same module. Module A did "with
>> Perforce.GlobalOptions(client=client): B.function()"
>> B.function did not receive the new GlobalOptions because of this
>> problem. As a result important files on the original client were
>> overwritten (OUCH).
>> I would like to propose that it be made impossible in the Python
>> source to import two instances of the same module.
> Impossible's a pretty strong word.
> It's a reasonable request, but with Python's importing the way it is
> it'd be kind of hard to do.  A Python file can be visible in multiple
> ways.
> However, anyone who does "import __init__" (or "from . import
> __init__" with relative import) is asking for trouble, I can't think
> of any valid reason to do it, and I wouldn't mind seeing that
> forbidden, but it's simple to avoid.  Someone probably did that
> because they didn't know how to import a containing package from one
> of its modules, failing to realize that it created a new module.
> Carl Banks
> --

There are corner cases. The corner case that I ran into was that there
were two ways to find the module on PATH because one value of PATH was
over another. Since then this problem has been removed and it wasn't
too much trouble to work around - but finding the problem was a real

I am not intimately familiar with the import code and trust your
judgment that it is difficult. If people are in agreement that this
should be changed though it could be put in a list somewhere waiting
for some ambitious person to figure out the implementation, no?

Personally I think it would be worthwhile.

Zachary Burns
Aim - Zac256FL
Production Engineer (Digital Overlord)
Zindagi Games

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