Broken when piped to.

When piping to python script that uses prompt() everything breaks:

File "/home/user/lib/python3.6/site-packages/prompt_toolkit/input.py", line 67, in __init__
    assert self.stdin.isatty()
AssertionError

To reproduce:

#test.py
from prompt_toolkit import prompt
text = prompt('> ')

Then:

$ echo "foo" | python test.py
<...>
File "/home/user/lib/python3.6/site-packages/prompt_toolkit/input.py", line 67, in __init__
    assert self.stdin.isatty()
AssertionError

[jonathanslenders]

Yes, a Posix pipe is not the same as a PTY (pseudo terminal). Both are I/O devices, but they have different properties.

• For instance, for a pipe between two processes, you want to have a big buffer, because otherwise, you end up switching between these process all the time. For user interaction, you want immediate feedback, so no buffering at all.
• Further, prompt_toolkit will discard the input I/O when too much was read from stdin and the input gets accepted (enter is pressed). You will loose this input data if you would pipe certain input. For user interaction, this doesn’t matter, because after pressing “enter”, the user waits to see the feedback. (The reason this works that way, is because it’s the only way to make processing pasted text on stdin efficient. - There is no way to read until a “newline” character, and there is also no way to push stdin, back into the input device. And reading one character at a time during paste events is slow.)
• A pipe does not respond to a CPR (cursor position request). There is actually no cursor.
• A pipe does not have raw or canonical input mode.

Probably there are other differences. There is a lot of non-obvious low level I/O underneath. But in any case, prompt_toolkit won’t support pipes as stdin. Maybe with some hacks it will work partly, but seriously, it’ll take only a few lines of code to read stdin yourself, if the input doesn’t appear to be a tty.

if not sys.stdin.isatty():
    data = sys.stdin.read()
    ...

[jonathanslenders]

Hi @Granitosaurus,

Thanks for reporting the issue. This is however something which won’t be fixed. (I guess I definitely need to given better error message as feedback.)

The reason is that prompt_toolkit is meant for user interaction, not for machine-interaction like a pipe. If you want to support pipe input on the other hand, it’s best to test for sys.stdin.isatty() yourself, and if that is true, then use sys.stdin.read/readline to retrieve the input.

The same is true for piping the output to something else. If you’d pipe stdout to a file, the result will be a meaningless sequence of ANSI escape characters. If sys.stdout.isatty() is true, don’t use prompt_toolkit.