When is stdin not connected to a tty, but can still be used interactively?

[stevendaprano]

I'm looking at some code which is intended to be used interactively, and I don't quite understand it.

Simplified, the code looks like this:

import sys

def example():
    try:
        import tty
        fd = sys.stdin.fileno()
        old = tty.tcgetattr(fd)
        tty.setcbreak(fd)
        getchar = lambda: sys.stdin.read(1)
    except (ImportError, AttributeError, io.UnsupportedOperation):
        # Fallback for when the terminal is not a tty.
        tty = None
        getchar = lambda: sys.stdin.readline()[:-1][:1]
    try:
        while True:
            c = getchar()
            if c in ('q', 'Q'):
                break
    finally:
        if tty:
            tty.tcsetattr(fd, tty.TCSAFLUSH, old)

The code waits until the user types 'q' or 'Q'. What I don't get is under what circumstances can you have a terminal session, complete with stdin, but *not* be attached to a tty?

Can somebody explain the circumstances where the fallback code will run please?

Thanks in advance.

[Gribouillis]

The program does not say that you have a terminal session when you are not attached to a tty. When it is not attached to a tty, it simply reads the standard input line by line and it takes the first character of each line as if the user had typed it in a terminal.

This allows automating input by the means of a pipe for example

Output:
$ simulate_keystrokes | python program.py