I need to manipulate large numbers in Python that fit into 64 bits. Currently, my code is running on a 64-bit platform but there is small but distinct possibility that it will have to run on a 32-bit platform.  Consequently, I would prefer to use long type to represent my numbers. I understand there is a performance impact for using long over int type. How bad is it? I'll be performing a lot of divisions and multiplications on them, but the results should all fit into 64 bits, too.
If you're going to be doing a lot of heavy number crunching, have a look at "numpy".
Performance impact of using long vs. int in Python