Agreed with Amaury. Module import could fail for various reasons (perhaps the same ones which led to the exception being raised!), or could deadlock if the import lock is being held. Also, having __str__ fail is usually very annoying for users (especially when it's the __str__ of an exception object).

If it's too hard to import errno at startup (because of bootstrapping), I would suggest using PyImport_ImportModuleNoBlock() instead, and silence errors (just print the numeric value of errno instead).