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

[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