How to override stdin for tests simulating user input? #9954

DavesCodeMusings · 2022-11-14T22:24:29Z

DavesCodeMusings
Nov 14, 2022

I've got an interactive program that I'd like to write some tests for. It requires simulating user input, so I'm trying to override stdin to pass pre-defined input. Unfortunately, I've succeeded only in crashing the system so far.

I started with this, based on examples I found on the interwebs:

import sys
from io import StringIO

with StringIO('Hello World') as f:
    stdin_save = sys.stdin
    sys.stdin = f
    msg = input()
    print('"{}" was passed to input() by overriding stdin.'.format(msg))
    sys.stdin = stdin_save

It works great in CPython, but fails in MicroPython. I think I know why, but I'm unsure how to fix it.

In CPython, sys.stdin is a TextIOWrapper

>>> import sys
>>> sys.stdin
<_io.TextIOWrapper name='<stdin>' mode='r' encoding='utf-8'>

But in MicroPython it's FileIO

>>> import sys
>>> sys.stdin
<io.FileIO 0>

So I tried changing it:

import sys
from io import FileIO

with FileIO('Hello World') as f:
    stdin_save = sys.stdin
    sys.stdin = f
    msg = input()
    print('"{}" was passed to input() by overriding stdin.'.format(msg))
    sys.stdin = stdin_save

Now I've made things worse and running the code above causes an unhandled exception that reboots the board. I also tried getting more specific with sys.stdin.read, but again I'm just crashing the board.

Not exactly where to go next. I'm hoping someone has done this kind of thing before and can point me in the right direction. Or is there another way to send simulated user input to a MicroPython program's stdin?

jimmo · 2022-11-25T05:42:09Z

jimmo
Nov 25, 2022
Maintainer

@DavesCodeMusings I think you can use os.dupterm to achieve this in MicroPython (this is how things like WebREPL inject stdin/stdout). See also https://github.com/micropython/micropython/blob/master/examples/bluetooth/ble_uart_repl.py which does the same thing for BLE.

1 reply

DavesCodeMusings Nov 27, 2022
Author

Thank you. I will give it a try.

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How to override stdin for tests simulating user input? #9954

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jimmo Nov 25, 2022 Maintainer

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Nov 14, 2022

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