Python os.waitpid() method is an efficient way for a process to wait for the completion of one or more of its child processes, as specified by the input parameter. The functionality of this method differs between UNIX and Windows OS. Syntax of Python os.waitpid() Method os.waitpid(pid, options) Parameters pid It is an integer value representing the ID of the child process to wait for. On Windows OS, any pid less than or equal to 0 raises an exception.