Where does st.file_uploader store uploaded files and when do they get deleted?
When you upload a file using st.file_uploader, the data are copied to the Streamlit backend via the browser, and contained in a BytesIO buffer in Python memory (i.e. RAM, not disk). The data will persist in RAM until the Streamlit app re-runs from top-to-bottom, which is on each widget interaction. If you need to save the data that was uploaded between runs, then you can cache it so that Streamlit persists it across re-runs.
As files are stored in memory, they get deleted immediately as soon as they’re not needed anymore.
This means Streamlit removes a file from memory when:
The user uploads another file, replacing the original one
The user clears the file uploader
The user closes the browser tab where they uploaded the file
