Due to the GIL (Global interpreter lock) only one instance of the python interpreter executes in a single process. So in general, using multi-threading only improves IO bound computations, not CPU-bound ones. The multiprocessing module is recommended if you wish to parallelise CPU-bound tasks.
GIL applies to CPython, the most popular implementation of Python, as well as PyPy. Other implementations such as Jython and IronPython have no GIL.