That's because Python's data structures aren't thread-safe. Indeed, only one data structure is guaranteed to be thread safe—the Queue class in the multiprocessing module. Queues are FIFOs (that is, ...

For parallelism, Python offers multiprocessing, which launches multiple instances of the Python interpreter, each one running independently on its own hardware thread.. All three of these ...

Post the results for each row to a multiprocessing.Queue, and spawn a single process that gets from the queue and writes to the file. It'll post some code when I get to work. More options ...