The example in https://docs.python.org/3/library/pty.html that mimics script(1) can be used to reproduce the problem: since window size is not set by pty.spawn, output of "ls" becomes scattered and hard to visually parse if xterm window is not in fullscreen mode.

To fix the above issue, this patch makes pty.spawn set window size ( TIOCSWINSZ ). Also, this patch makes the code of pty.fork() more readable by defining _login_pty(); the latter is reused in spawn().