[Python-ideas] Prevent importing yourself?

[Ned Batchelder]

On 1/30/16 5:47 PM, Steven D'Aprano wrote:
> On Sat, Jan 30, 2016 at 06:19:35AM -0500, Ned Batchelder wrote:
>
>> While we're at it though, re-importing __main__ is a separate kind of
>> behavior that is often a problem, since it means you'll have the same
>> classes defined twice.
> As far as I can tell, importing __main__ is fine. It's only when you
> import __main__ AND the main module under its real name at the same time
> that you can run into problems -- and even then, not always. The sort of
> errors I've seen involve something like this:
>
> import myscript
> import __main__  # this is actually myscript
> a = myscript.TheClass()
> # later
> assert isinstance(a, __main__.TheClass)
>
> which fails, because myscript and __main__ don't share state, despite
> actually coming from the same source file.
>
> So I think it's pretty rare for something like this to actually happen.
> I've never seen it happen by accident, I've only seen it done
> deliberately as a counter-example to to the "modules are singletons"
> rule.
>
>

Something like this does happen in the real world.  A class is defined 
in the main module, and then the module is later imported with its real 
name.  Now you have __main__.Class and module.Class both defined.  You 
don't need to actually "import __main__" for it to happen. 
__main__.Class is used implicitly from the main module simply as Class.

--Ned.