Web Application Testing

To test local web applications, write native Python Playwright scripts. Your goal is to extract the information you need without polluting context with huge amounts of data (the entire DOM, every selector on the page, etc).

patterns

use the decision tree

User task → Is it static HTML?
    ├─ Yes → Read HTML file directly to identify selectors
    │         ├─ Success → Write Playwright script using selectors
    │         └─ Fails/Incomplete → Treat as dynamic (below)
    │
    └─ No (dynamic webapp) → Is the server already running?
        ├─ No → Run: python scripts/with_server.py --help
        │        Then use the helper + write simplified Playwright script
        │
        └─ Yes → Reconnaissance-then-action:
            1. Navigate and wait for networkidle
            2. Take screenshot or inspect DOM
            3. Identify selectors from rendered state
            4. Execute actions with discovered selectors

act programmatically, then search for precise information

1. Inspect rendered DOM:

   page.screenshot(path='/tmp/inspect.png', full_page=True)
   content = page.content()
   page.locator('button').all()
   

2. Identify selectors from inspection results
3. Execute actions using discovered selectors

scripts

• scripts/with_server.py - Manages server lifecycle (supports multiple servers)

with_server.py

To start a server, run --help first, then use the helper:

Single server:

python scripts/with_server.py --server "npm run dev" --port 5173 -- python your_automation.py

Multiple servers (e.g., backend + frontend):

python scripts/with_server.py \
  --server "cd backend && python server.py" --port 3000 \
  --server "cd frontend && npm run dev" --port 5173 \
  -- python your_automation.py

To create an automation script, include only Playwright logic (servers are managed automatically):

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True) # Always launch chromium in headless mode
    page = browser.new_page()
    page.goto('http://localhost:5173') # Server already running and ready
    page.wait_for_load_state('networkidle') # CRITICAL: Wait for JS to execute
    # ... your automation logic
    browser.close()

rules

• Never inspect the DOM before waiting for networkidle on dynamic apps
• Always wait for page.wait_for_load_state('networkidle') before inspection
• Never execute actions which load large amounts of data into the context. Instead, act programmatically and search for what you need (e.g. DOM elements).
• Always check the scripts available in scripts/ before doing a task. Using the scripts keeps the context window clean, which is ultra critical.
• Prefer to not read the source of the bundled scripts. Instead, invoke them with --help to avoid polluting context.
• Always invoke a script with --help to see usage before attempting to use.
• Always use sync_playwright() for synchronous scripts
• Always close the browser when done
• Always use descriptive selectors: text=, role=, CSS selectors, or IDs
• Always add appropriate waits: page.wait_for_selector() or page.wait_for_timeout()

references

• examples/ - Examples showing common patterns:
  • element_discovery.py - Discovering buttons, links, and inputs on a page
  • static_html_automation.py - Using file:// URLs for local HTML
  • console_logging.py - Capturing console logs during automation