ImportError with pickle (Python 2.7.9), possibly platform dependent

[Ben Sizer]

On Thursday, 30 April 2015 01:45:05 UTC+1, Chris Angelico  wrote:
> On Thu, Apr 30, 2015 at 2:01 AM, Ben Sizer wrote:
> > 1) There clearly is a module named OMDBMap, and it's importable - it's there in the 2nd line of the traceback.
> >
> > Does anybody have any suggestions on how I can go about debugging this? Or refactoring it to avoid whatever is happening here?
> 
> Are you half way through importing it when this load() call happens?
> That might cause some issues.

No, we already imported OMDBMap at the top of OMDBSetup.

> Has your current directory been changed anywhere in there?

Good question. It turns out that the current directory seems to be $HOME when loading, but is the script directory during saving. This will be because the Linux server is running under mod_wsgi, whereas we run the save script in-place. Our Windows and Mac tests run via Flask's built-in server so the working directory is likely to be the same whether we're running the script that does pickle.dump or the whole app that does pickle.load.

> What happens if you catch this exception and print out sys.modules at
> that point?

Another good question, and this gives us the answer. The module lists are quite different, as I'd expect because the load happens in the context of the full application whereas the dump happens as a standalone script. But literally every module that is in the 'before dump' list is in the 'before load' list - except OMDBMap. Like the error says! What /is/ in the 'before load' list however is "my_wsgi_app.OMDBMap". The module has been imported, but the pickle algorithm is unable to reconcile the module in the WSGI app's namespace with the module referenced in the pickle file.

So... I don't know how to fix this, but I do now know why it fails, and I have a satisfactory answer for why it is acting differently on the Linux server (and that is just because that is the only one running under WSGI). Two out of three isn't bad!

Thanks,
-- 
Ben Sizer