如何使用带有 on_click 回调的 st.download_button 下载前处理数据？

Question

I have an app running where my model gives result a np.ndarray and I'm showing the results as st.image(result_matrix) . I want to add a functionality where I want to give my users the ability to download this image but the problem is that I have to convert to that to PIL.Image and send the buffer.getvalue() as input to this button. I can do this too but my users don't download very often and to save the computation power and load, I'm not converting EVERY result to PIL.Image .

Is there any functionality where you can download the data, after processing it, on demand?

I tried doing the below but gave me obvious error that it doesn't accept numpy array:

import streamlit as st
from PIL import Image
import numpy as np
from io import BytesIO

st.session_state['result'] = some_numpy_RGB_array

def process_image():
    img = Image.fromarray(st.session_state['result'])
    buffer = BytesIO()
    img.save(buffer, format="jpeg")
    st.session_state['result'] = buffer.getvalue()

_ = st.download_button(label="Download",data=st.session_state['result'],file_name="image.jpeg",mime="image/jpeg",on_click=process_image)

Answer 1

I'm only aware of the workaround given here :

import streamlit as st


def generate_report(repfn):
    with open(repfn, 'w') as f:
        f.write('Report')
    st.write('done report generation')


if st.button('generate report'):
    repfn = 'report.pdf'
    generate_report(repfn)

    with open(repfn, "rb") as f:
        st.download_button(
            label="Download report",
            data=f,
            file_name=repfn)

It's not ideal, because the user has to click two buttons: one to generate the (in your case) image, and a second one to actually download it. But I guess it's better than nothing.