debugging-streamlit

name debugging-streamlit description Debug Streamlit frontend and backend changes using make debug with hot-reload. Use when testing code changes, investigating bugs, checking UI behavior, or needing screenshots of the running app. Debugging Streamlit Apps Quick Start make debug my_app.py This starts both backend (Streamlit/Python) and frontend (Vite/React) with hot-reload. The app URL is printed on startup (default http://localhost:3001 ; 3000 is reserved for manual make frontend-dev ; it may use 3002+ if other debug sessions are running). Avoid pinning VITE_PORT unless you have a specific hard requirement (last resort). Hot-reload behavior: Frontend : Changes to frontend/ code are applied within seconds. Backend : Only changes to the app script trigger a rerun. Changes to the Streamlit library itself ( lib/streamlit/ ) require restarting make debug . Log Files Each make debug run writes logs to a per-session directory under work-tmp/debug/ and updates work-tmp/debug/latest/ to point at the most recent session. Because latest/* is a symlink, it can move if multiple debug sessions are starting/stopping concurrently—prefer using the session directory path printed by make debug when you need stable log references. You can find the exact session directory in the make debug startup output under the Log files section. File Content work-tmp/debug/latest/backend.log Python print() statements, Streamlit logs, errors work-tmp/debug/latest/frontend.log Browser console.log() , React errors, Vite output Logs are cleared at the start of each session and persist after exit for post-mortem analysis. Log size warning: Logs can grow large during extended debugging sessions. Instead of reading entire log files, use rg to search for specific patterns: # Search for specific debug messages rg "DEBUG:" work-tmp/debug/latest/backend.log # Search for errors (case-insensitive) rg -i "error|exception|traceback" work-tmp/debug/latest/backend.log # Search with context (3 lines before/after) rg -C 3 "my_function" work-tmp/debug/latest/backend.log # Search frontend logs for specific component rg "MyComponent" work-tmp/debug/latest/frontend.log Use this directory for all debugging artifacts (scripts, screenshots, etc.) to keep them organized. Adding Debug Output Backend (Python): print ( f"DEBUG: session_state = {st.session_state} " ) Frontend (TypeScript/React): console . log ( "DEBUG: props =" , props) Frontend console.log() output appears in work-tmp/debug/latest/frontend.log (or the current session's frontend.log file). Workflow Create or use a test script in work-tmp/debug/ (e.g., work-tmp/debug/test_feature.py ) Run make debug work-tmp/debug/test_feature.py Verify startup : Check work-tmp/debug/latest/backend.log for Error / Exception and work-tmp/debug/latest/frontend.log for console errors to ensure both servers started correctly Access the printed App URL in your browser (default http://localhost:3001 , but it may be 3002+ ) Verify script execution : Check work-tmp/debug/latest/backend.log again for any errors after the first app access Monitor logs by inspecting work-tmp/debug/latest/backend.log and work-tmp/debug/latest/frontend.log Edit code - changes apply automatically via hot-reload Check logs for debug output Quick error check: # Backend errors rg -i "error|exception" work-tmp/debug/latest/backend.log # Frontend console errors rg -i "error" work-tmp/debug/latest/frontend.log Temporary Playwright Scripts for Screenshots & Testing For advanced debugging with screenshots or automated UI interaction. Quick: Playwright CLI For simple screenshots and interactions, use @playwright/cli (available in frontend devDependencies): cd frontend STREAMLIT_APP_URL=http://localhost:3001 yarn playwright-cli open " $STREAMLIT_APP_URL " yarn playwright-cli screenshot --filename ../work-tmp/debug/screenshot.png --full-page yarn playwright-cli close See https://github.com/microsoft/playwright-cli for more commands ( snapshot , click , fill , etc.). Custom Scripts For complex interactions, create temporary Playwright scripts in work-tmp/debug/ : # work-tmp/debug/debug_screenshot.py """Temporary Playwright script for debugging - run against make debug.""" import os from playwright.sync_api import sync_playwright, expect from e2e_playwright.shared.app_utils import get_text_input, click_button from e2e_playwright.conftest import wait_for_app_loaded, wait_for_app_run def main (): app_url = os.environ.get( "STREAMLIT_APP_URL" , "http://localhost:3001" ) with sync_playwright() as p: browser = p.chromium.launch(headless= True ) page = browser.new_page(viewport={ "width" : 1280 , "height" : 720 }) # Connect to app started with `make debug` page.goto(app_url) wait_for_app_loaded(page) # Interact with the app text_input = get_text_input(page, "Name" ) text_input.fill( "Test User" ) click_button(page, "Submit" ) wait_for_app_run(page) # Verify and screenshot expect(page.get_by_text( "Hello, Test User" )).to_be_visible() page.screenshot(path= "work-tmp/debug/debug_screenshot.png" , full_page= True ) print ( "Screenshot saved to work-tmp/debug/debug_screenshot.png" ) browser.close() if __name__ == "__main__" : main() Running Temporary Scripts Ensure make debug <app.py> is running first (start it in a background task if needed). If your make debug session is using a non-default port, set STREAMLIT_APP_URL accordingly, then run the Playwright script: STREAMLIT_APP_URL=http://localhost:3001 \ PYTHONPATH=. uv run python work-tmp/debug/debug_screenshot.py This uses the uv-managed environment with all dependencies (playwright, etc.) and makes e2e_playwright importable without path manipulation. Available Utilities from e2e_playwright Element Locators & Interactions ( e2e_playwright.shared.app_utils ): Provides helpers like get_text_input() , get_button() , click_button() , get_checkbox() , etc. Synchronization ( e2e_playwright.conftest ): wait_for_app_loaded(page) - wait for initial load wait_for_app_run(page) - wait for script execution after interaction wait_until(page, fn, timeout) - poll until condition is true Playwright API Reference : https://playwright.dev/python/docs/api/class-playwright Screenshot Best Practices # Full page screenshot page.screenshot(path= "work-tmp/debug/full.png" , full_page= True ) # Element screenshot element = page.get_by_test_id( "stDataFrame" ) element.screenshot(path= "work-tmp/debug/dataframe.png" ) Troubleshooting Port already in use / multiple sessions: make debug will automatically pick a free frontend port (typically in the 3001-3100 range) so multiple debug sessions can run simultaneously. Frontend port 3000 is reserved for manual make frontend-dev sessions. If you have a hard requirement for a specific frontend port, you can pin it with VITE_PORT=3002 make debug <app.py> (last resort). Hot-reload not working: Backend: Only the app script is watched. Changes to lib/streamlit/ require restarting make debug . Frontend: Check work-tmp/debug/latest/frontend.log for Vite errors. TypeScript errors can break HMR. Playwright script fails to connect: Verify make debug is running and healthy Check the printed App URL is accessible in the browser Ensure wait_for_app_loaded(page) is called after page.goto() Cleanup After debugging is complete, remove temporary scripts and screenshots from work-tmp/debug/ . Related Skills understanding-streamlit-architecture skill : For deeper understanding of backend/frontend internals when debugging cross-layer issues