Help with cache, callbacks or session state

Using Streamlit
1

Hello community,

I’m trying to implement some sort of memory into my app, but I’m struggling to understand exactly how to do it and which component I should use.

Here’s how it works:
I have a multipage app, done with the latest native support from streamlit.
On each page, I can drag&drop a file (different type of file on each page but always a .csv, log, or text file). The drag & drop widget is on the page, not on a sidebar.
On each page, the uploaded file gets analyzed, some calculations are performed then a bunch of plots are made.

The issue is that for now, when I switch to another page, I loose the results (file and results) on the previous page I was on and need to re-upload the input file if I want to see the results again.

I have a sidebar with either links to markdowns or widgets (sliders or info box).

By reading the documentation, I feel like the session state is not what I should use, but more the cache function.

Any help on how I could implement that, to make my app more user friendly?

2

Multipage and session_state do not yet seem to play too well together: widget state not synced between multiple pages · Issue #4989 · streamlit/streamlit · GitHub . One can work around this by creating extra variables not related to a widget but this somewhat messes the code… I also hope this gets resolved soon.

3

To clarify, the issue that is reported in that Github issue is not about session_state per se, but rather about session state entries that come automatically from widgets (e.g. if I do st.checkbox(..., key=123) that automatically generates an entry in st.session_state called 123, but that session state entry might not persist across pages).

That being said, my recommendation would be:

  1. Use session_state to keep track of the files that have been uploaded
  2. Show the files that have previously been uploaded in the sidebar. If a user then uploads a new file, overwrite that file.
  3. If you need to do something with the file, then you can access it from st.session_state on any page.

Here are two examples of what that might look like:

streamlit_app.py:

from io import StringIO

import pandas as pd
import streamlit as st

my_csv = st.file_uploader("Upload csv", type="csv")

if my_csv is not None:
    st.session_state["my_csv"] = my_csv.getvalue().decode("utf-8")

if "my_csv" in st.session_state:
    st.sidebar.dataframe(pd.read_csv(StringIO(st.session_state["my_csv"])))

pages/page1.py

import streamlit as st

my_image = st.file_uploader("Upload image", type="jpg")

if my_image is not None:
    st.session_state["my_image"] = my_image

if "my_image" in st.session_state:
    st.sidebar.image(st.session_state["my_image"])

Note that if you switch pages, the uploaded files are still saved, and don’t need to be re-uploaded.

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