file_uploader does not repopulate the UploadedFile stream when the app is rerun

Summary

I use st.file_uploader to allow the user to submit their own data. Then I do an action based on this data and other user-provided parameters (coming from an st.slider for instance). Re-running the app produces an error. This includes when the parameters are changed (for example by moving the st.slider). After investigating, I discovered that it is because, after the first reading of the UploadedFile output by st.file_uploader, the UploadedFile stays empty. It is not re-populated by the uploaded file in spite of the app re-running.

Steps to reproduce

1. Save the following JSON as “test.json”:

{"a": 3, "b": 2}

1. Save the following piece of code as “my_app.py” (or any other name):

import json
import streamlit as st

st.title("Let's add a number to the elements of a json")

uploaded_file = st.sidebar.file_uploader(
    "Upload your json", type=["json"]
)

to_add = st.sidebar.slider(
    "Choose a number", min_value=1, max_value=5, value=3
)

st.markdown("The result is:")

default_file = open("test.json", "r")
if uploaded_file is None:
    dico = json.load(default_file)
else:
    dico = json.load(uploaded_file)
st.write({k: v + to_add for k, v in dico.items()})

default_file.close()

1. Run streamlit run my_app.py
2. In the app, upload test.json in the file uploader. You should see that:
3. Move the slider, here is the bug, you should see that:
4. At this point, re-uploading works, but the user shouldn’t have to re-upload the file after each slider change or each plain re-run. Especially since, as we see in the screenshot above, the widget still displays the informations of the uploaded file. Here is how it looks after re-uploading:

Expected behavior:

We shouldn’t ever get to the second screenshot. We should pass from the first to the third simply by moving the slider and without re-uploading the file every time.

Actual behavior:

As explained above, after the first run the stream is empty and not reset when the app is re-run. This is not specific to the JSON format, although the error message displayed above is. I have run the same test with a CSV file and pandas.read_csv. The underlying cause is the same: an empty stream.

Note that I have also tried to make an auxiliary function that encapsulates the dico = json.load(uploaded_file) part and which is decorated by st.cache. It still doesn’t work.

Is this a regression?

I don’t know, it’s the first time I use st.file_uploader.

Debug info

I have two different settings.

An AWS distant machine (I forward the port to my local machine):

• Streamlit version: 0.69.1
• Python version: 3.7.6
• Using Conda
• OS version: I don’t know

My local machine:

• Streamlit version: 0.68.0
• Python version: 3.8.5
• Not using Conda
• OS version: macOS 10.13.6
• Browser version: Firefox 81.0.1

Additional information

Side note on the documentation of st.file_uploader: at the some point it says to use StringIO(uploaded_file.decode("utf-8")) but this doesn’t work and raises AttributeError: 'UploadedFile' object has no attribute 'decode'. Using StringIO(uploaded_file.read().decode("utf-8")) does work, though.

[FrankKr]

Exalate commented: FrankKr commented: Update: Actually, it appears the UploadedFile stream is available, but the cursor is at the end of the stream after the first evaluation. A work-around for me was to reset the cursor by including uploaded_file.seek(0) after the check if uploaded_file is None. Would very much prefer this workaround to become obsolete.

in your example:

default_file = open("test.json", "r")
if uploaded_file is None:
    uploaded_file.seek(0)
    dico = json.load(default_file)
else:
    dico = json.load(uploaded_file)

st.write({k: v + to_add for k, v in dico.items()})

default_file.close()

original: experiencing the same issue after upgrading streamlit from version 0.64.0 to 0.69.0

[karriebear]

Closing this issue as a duplicate of #2235. A fix for this should be coming in the next release. Please track #2235 for updates.