Subprocess A running program is called a process . Each process has its own system state, which includes memory, lists of open files, a program counter that keeps track of the instruction being executed, and a call stack used to hold the local variables of functions. Normally, a process executes statements one after the other in a single sequence of control flow, which is sometimes called the main thread of the process. At any given time, the program is only doing one thing. A program can create new processes using library functions such as those found in the os or subprocess modules such as os.fork() , subprocess.Popen() , etc. However, these processes, known as subprocesses , run as completely independent entities-each with their own private system state and main thread of execution. Because a subprocess is independent, it executes concurrently with the original process. That is, the process that created the subprocess can go on to work on other things while the subproc