stdin

[kkoomen]

Hi,

I've tried to get stdin working but without any success, unfortunately. I've tried the following:

import sys

def get_input(prompt):
    sys.stdout.write(prompt)
    sys.stdout.flush()
    return sys.stdin.readline().rstrip()

x = get_input('Type something: ')
print(x)

but this still requests for input and then prints it, while I expect to first print what's been written to the stdout and then request for input. Is this possible with the latest version of Pyodide?

[ryanking13]

Thanks for the report. I think you can additionally do os.fsync(sys.stdout.fileno()) to ensure the data is written (ref) to the file (in this case, stdout).

[kkoomen]

Ah, so what I do inside our project is: I have an editor and the user can type in some Python code and run it. When it will be executed, I always run sys.stdout = io.StringIO() first, then execute the code and then run sys.stdout.getvalue(), but I guess that's not an option? How would I use input() without the use of sys.stdout = io.StringIO()? Any recommendations?

[ryanking13]

How would I use input() without the use of sys.stdout = io.StringIO()

Well, input() is not related to stdout but stdin, so I am unsure if I understood your question correctly.

Anyway, we provide some way to redirect streams: https://pyodide.org/en/stable/usage/streams.html, so you can redirect stdin and stdout. I guess this document will help?

[kkoomen]

@ryanking13 Okay so let me just clarify what I actually want in pyodide.

I want to run this script exactly the same in pyodide as if I were to run this in a python3 REPL or script:

print('Welcome')
num1 = input('Enter a number: ')
print('Thank you!')
num2 = input('Enter another number: ')
print(f'Your two numbers are: {num1} and {num2}')

but currently our version outputs this:

Screen.Recording.2024-04-28.at.13.39.54.mov

FYI: we're using pyodide inside a worker. Here's the relevant code: https://github.com/uvapl/examide/blob/feature/stdin-py/static/js/workers/py.worker.js#L20-L69

Any idea on how to make this work properly? Ideally, the output should be this:

Welcome
Enter a number: 123
Thank you!
Enter another number: 456
Your two numbers are: 123 and 456

[ryanking13]

The input() calls fflush to flush the batched output, but it seems like that is not enough, and you need to call fsync to actually apply the output to disk (in this case, stdout). TBH, I don't really understand the difference between the two, but there seems a difference (emscripten-core/emscripten#14457 (comment)).

So I think we have two options here:

1. Use "unbatched" stdout.

By default, when you pass stdout parameter when loading pyodide. We use "batch" mode. That is, the data is only flushed when it gets "\n".

We have a "raw" mode that does not use batching, which I think you can have a look.

// see: https://pyodide.org/en/stable/usage/api/js-api.html#pyodide.setStdout
pyodide.setStdout({ raw: handler })

2. Find a way to call fsync inside a input.

You can override the builtin input function to call os.fsync(sys.stdout.fileno()).

[kkoomen]

Oh! that indeed fixed it! Awesome, thanks a lot for your time, I appreciate it!

py-demo.mov