Return str to a callback raise a segfault if used in string formating

[Vincent Vande Vyvre]

Simplified code:

---%<------------------------------------------
class Foo:
     tasks = []
     def process(self, *args):
         # process with the file
         # save result on disk with a new name
         self.out_filename.write(.....)

     def run_multitasks(self):
         while self.tasks:
             t = Thread(target=process, args=self.tasks.pop(0))
             t.start()
             t.join()
             self.callback(self.out_filename)

     def add_task(self, fname, callback):
         self.tasks.append(fname)
         self.callback = callback
         if len(self.tasks) == 1:
             self.run_multitasks()

def slot(filename):
     print(filename)                 # Works
     print("%s created" % filename)  # Segfault !

Files = [list of files]
foo = Foo()
for f in Files:
     foo.add_task(f, slot)
---%<------------------------------------------

If I place self.callback() at the end of the func process that doesn't 
change anything.


Regards,

Vincent.