Hi Martin, Normally, Python caches modules after they are imported, so when you import a module for the second time it doesn't actually execute the code again. If you have 3 modules, "one", "two" and "common", and "one" and "two" both import "common", "common" should only be loaded once.
However, there are a few situations where this can break. One is when the initial script for your application (the one that you run from the command line) is also imported by other modules. Typically the script will be loaded twice, once with the name __main__, and once with its "real" name. Other possibilities might be import loops, certain uses of "from <module> import <something>", manipulation of PYTHONPATH/sys.path etc. It's difficult to know what the cause is without seeing the code. See also http://python-notes.curiousefficiency.org/en/latest/python_concepts/import_traps.html#the-double-import-trap. By looking at the value of __name__ and the stack traces when you get exceptions, you might be able to figure it out. http://stackoverflow.com/a/4897479/395053 suggests inserting "import pdb; pdb.set_trace()" above the class definition and using bt to display backtraces. In general, the solution is to ensure that your imports are structured as simply as possible and avoid circular dependencies. Hope that helps, Simon On Mon, Mar 2, 2015 at 11:57 AM, Martino Io <martino8...@gmail.com> wrote: > Hi Simon, definitely multiple imports is the issue here, however I don't > know how to get rid of it, > let's say I have a module which i named "ormstuff" inside it I have a class > named "mybase", I do have 2 other modules which > are using this class, and to do so I import it in both places. Now this > might just be me skipping the basics, however > if I don't import the mybase in both modules I cannot use it, since it > should throw an undefined variable exception. > > 2015-03-02 12:47 GMT+01:00 Simon King <si...@simonking.org.uk>: >> >> On Mon, Mar 2, 2015 at 11:29 AM, Martino Io <martino8...@gmail.com> wrote: >> > Hello, I've been busy writing an application which now has grown >> > considerably, it spans across several packages/modules totalling 200K >> > lines >> > of python code. >> > Recently I made some changes to the structure and decided to split >> > several >> > classes into separate modules for greater flexibility; the problem I'm >> > facing now >> > is quite strange, before I was using mainly 1 file for all the ORM >> > definitions, but since I split that into several files, I've started to >> > get >> > exceptions due to multiple >> > Base objects being defined and no shared mapper to store the metadata. >> > I've >> > changed approach and started to import the base from a single module, >> > however given the high number of modules and cross imports I get >> > exceptions >> > like: >> > >> > sqlalchemy.exc.InvalidRequestError: Table 'fabrics' is already defined >> > for >> > this MetaData instance. Specify 'extend_existing=True' to redefine >> > options >> > and columns on an existing Table object. >> > >> > I thought that by adding "use_existing" would fix the issue (the mapper >> > will ignore further imports which redefine the objects) but instead got: >> > >> > InvalidRequestError: Multiple classes found for path "Group" in the >> > registry >> > of this declarative base. Please use a fully module-qualified path. >> > >> > So going back to the question, if I have a cross dependant package >> > (which >> > imports classes which are ORM classes), how do I avoid the mapper >> > "redefining" a class? >> > Also any advice on best way to structure packages/modules in an ORM use >> > case >> > would be appreciated. >> > >> >> extend_existing is almost certainly not what you want here. Each >> mapped class should only be defined in a single place. If you've done >> that, and are still getting this error, it suggests that perhaps your >> Python modules are being imported more than once. Try putting "print >> 'importing module %s' % __name__" before your class definition. If you >> see the output more than once, then the module is being imported >> multiple times under different names. >> >> Hope that helps, >> >> Simon >> >> -- >> You received this message because you are subscribed to a topic in the >> Google Groups "sqlalchemy" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/sqlalchemy/gExzW1RNF7A/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> sqlalchemy+unsubscr...@googlegroups.com. >> To post to this group, send email to sqlalchemy@googlegroups.com. >> Visit this group at http://groups.google.com/group/sqlalchemy. >> For more options, visit https://groups.google.com/d/optout. > > > -- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at http://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. 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