I am working with an ARM Cortex M3 on which I need to port Python (without operating system). What would be my best approach? I just need the core Python and basic I/O.
There are a few projects that have attempted to port Python to the situation you mention, take a look at python-on-a-chip, PyMite or tinypy. These are aimed at lower power microcontrollers without an OS and tend to focus on slightly older versions of the Python language and reduced library support.