What is the default rounding mode (rounding to nearest, etc) in Python? And how can we specify it?
I assume you are talking about the basic float type (double in C-like languages) and its operations.
With IEEE754-based platform (as most modern ones do, including x86, ARM, MIPS...), it's default mode "round to nearest, ties to even" is the only mode available in Python standard library. That is "provided" by standardized defaults and absense of library methods to change it. There are more languages that doesn't allow to change rounding mode - e.g. Java - so this isn't an isolated Python whim.
In real, there are too few reasons to change this. Direct rounding modes of IEEE754 are very special in their use. (I don't apologize the approach to stick on the default rounding, but simply comment on it.) For example, multiply of 1e308 by 1e308 with rounding to zero or to minus infinity results in approximately 1.8e308, so, the result is too far both from the exact answer and from POLA-based one (infinity). If you really need some specific modes for your computations, consider using specific libraries, like MPFR or gmpy2.
If you insist on changing this without external modules specialized on floating-point calculations, try using C-library fesetround via ctypes module or analog, e.g. here. Again, it's your choice to use such hacks and become responsible to all consequences. I'd suggest wrapping all pieces with special rounding to C-level code which restores the default mode on function exit.