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

1. Create or use a test script in work-tmp/debug/ (e.g., work-tmp/debug/test_feature.py)
2. Run make debug work-tmp/debug/test_feature.py
3. 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
4. Access the printed App URL in your browser (default http://localhost:3001, but it may be 3002+)
5. Verify script execution: Check work-tmp/debug/latest/backend.log again for any errors after the first app access
6. Monitor logs by inspecting work-tmp/debug/latest/backend.log and work-tmp/debug/latest/frontend.log
7. Edit code - changes apply automatically via hot-reload
8. 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