package import dangers

[Diez B. Roggisch]

Ethan Furman wrote:

> Greetings!
> 
> I'm working on a package with multiple modules (and possibly packages),
> and I would like to do it correctly.  :)
> 
> I have read of references to possible issues regarding a module being
> imported (and run) more than once, but I haven't been able to find
> actual examples of such failing code.
> 
> My google search was fruitless (although still educational !-), so if
> anyone could point me in the right direction I would greatly appreciate
> it.

The most common problem is that a file is used as module and as executable
at the same time.

Like this:

--- test.py ---

class Foo(object):
    pass


if __name__ == "__main__":
   import test
   assert Foo is test.Foo

---

This will fail when executed from the commandline because the module is
known twice - once as "__main__", once as "test".

So keep your startup-scripts trivial, or don't ever import from them.

You might create similar situations when modifying sys.path to reach *into*
a package - but that would be sick to do anyway.

Other than that, I'm not aware of any issues.

Diez