Windows lacks a fork() system call, which duplicates current process. This has many implications, including those listed on the windows multiprocessing documentation page. More specifically:
Bear in mind that if code run in a child process tries to access a global variable, then the value it sees (if any) may not be the same as the value in the parent process at the time that Process.start was called.
In internals, python creates a new process on windows by starting a new process from scratch, and telling it to load all modules again. So any change you have done in current process will not be seen.
In your example, this means that in the child process, your module will be loaded, but the if __name__ == '__main__' section will not be run. So T.init will not be called, and T.val won’t exist, thus the error you see.
On the other hand, on POSIX systems (that includes Linux), process creation uses fork, and all global state is left untouched. The child runs with a copy of everything, so it does not have to reload anything and will see its copy of T with its copy of val.
This also means that Process creation is much faster and much lighter on resources on POSIX systems, especially as the “duplication” uses copy-on-write to avoid the overhead of actually copying the data.
There are other quirks when using multiprocessing, all of which are detailed in the python multiprocessing guidelines.