Browser alerts pause Selenium tests because WebDriver cannot keep clicking or reading the page until the modal dialog is handled. A destructive-button test, confirmation prompt, or login warning can fail when the script tries to inspect the page while the alert still owns the browser focus.

Selenium exposes JavaScript alerts, confirmation dialogs, and prompt dialogs through an Alert object after the driver switches context. Waiting for the alert before reading it prevents a race between the click that opens the dialog and the browser creating the modal.

Use this flow for native browser dialogs created by alert(), confirm(), or prompt(). HTML modal components remain normal page elements, so handle those with locators and waits instead of switch_to.alert.

Related: How to install Selenium WebDriver for Python
Related: How to use explicit waits in Selenium
Related: How to run a headless browser in Selenium

Steps to handle browser alerts with Selenium:

  1. Create a local alert demo script. 
    $ cat > selenium-alert-handle.py <<'PY'
from pathlib import Path
from tempfile import TemporaryDirectory

from selenium import webdriver
from selenium.webdriver.chrome.options import Options
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait


HTML = """<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Selenium alert demo</title>
  </head>
  <body>
    <button id="delete" onclick="if (confirm('Delete this test record?')) {
      document.getElementById('state').textContent = 'accepted';
    } else {
      document.getElementById('state').textContent = 'dismissed';
    }">Delete test record</button>
    <p id="state">waiting</p>
  </body>
</html>
"""


options = Options()
options.add_argument("--headless=new")

with TemporaryDirectory() as tmpdir:
    page = Path(tmpdir) / "alert-demo.html"
    page.write_text(HTML, encoding="utf-8")

    driver = webdriver.Chrome(options=options)
    try:
        driver.get(page.as_uri())
        driver.find_element(By.ID, "delete").click()

        wait = WebDriverWait(driver, 5)
        alert = wait.until(lambda d: d.switch_to.alert)
        message = alert.text
        alert.accept()

        state = driver.find_element(By.ID, "state").text
        assert message == "Delete this test record?"
        assert state == "accepted"

        print(f"Alert text: {message}")
        print("Alert accepted")
        print(f"Page state: {state}")
    finally:
        driver.quit()
PY

    The demo uses a local file and a headless Chrome session so the alert behavior can be tested without touching a real application.

  2. Run the script to confirm Selenium reads and accepts the alert. 
    $ python3 selenium-alert-handle.py
Alert text: Delete this test record?
Alert accepted
Page state: accepted
  3. Use dismiss() when the test should choose the cancel path. 
    alert = wait.until(lambda d: d.switch_to.alert)
message = alert.text
alert.dismiss()

    Use accept() for OK or confirm actions, and dismiss() for Cancel on confirmation dialogs.

  4. Type into prompt dialogs before accepting them. 
    alert = wait.until(lambda d: d.switch_to.alert)
alert.send_keys("Selenium")
message = alert.text
alert.accept()
  5. Keep the alert wait directly after the action that opens the dialog in the real test. 
    driver.find_element(By.CSS_SELECTOR, "button.delete").click()
alert = wait.until(lambda d: d.switch_to.alert)
assert alert.text == "Delete this record?"
alert.accept()

    If a native alert is still open, normal element lookups and clicks can fail until accept() or dismiss() closes it.

  6. Remove the demo script if it was only used to verify the alert flow. 
    $ rm selenium-alert-handle.py