redirect STDIO in the Python code

[Gribouillis]

when any Python library forks a new process, does it make whatever is in sys.stdout, which may be using some other fd, such as fd 6, instead of fd 1, become fd 1

There is a lot of confusion here because you are mixing low-level concepts that belong to the C implementation of Python (such as file descriptors) with high-level concepts such as sys.stdin and sys.stdout which are abstract Python file objects that don't even need a file descriptor. The original question is too broad. If you want to do OS level operations in CPython, then use the available functions in modules such as os and subprocess. Most of what you do in C can be done in CPython, you are allowed to use functions such as os.fork or os.dup2, etc.