docs(python): add extension testing guide with fixtures (#15486)

docs/src/chrome-extensions-js-python.md

@@ -99,18 +99,17 @@ with sync_playwright() as playwright:
 
 To have the extension loaded when running tests you can use a test fixture to set the context. You can also dynamically retrieve the extension id and use it that to load and test the popup page for example.
 
-```typescript
-import { test as base, expect, chromium, BrowserContext } from "@playwright/test";
+```ts
+import { test as base, expect, BrowserContext } from "@playwright/test";
 import path from "path";
 
 export const test = base.extend<{
   context: BrowserContext;
   extensionId: string;
 }>({
-  context: async ({ browserName }, use) => {
-    const browserTypes = { chromium };
+  context: async ({ }, use) => {
     const pathToExtension = path.join(__dirname, "my-extension");
-    const context = await browserTypes[browserName].launchPersistentContext("", {
+    const context = await chromium.launchPersistentContext("", {
       headless: false,
       args: [
         `--disable-extensions-except=${pathToExtension}`,
@@ -148,3 +147,58 @@ test("popup page", async ({ page, extensionId }) => {
   await expect(page.locator("body")).toHaveText("my-extension popup");
 });
 ```
+
+```py
+# conftest.py
+from typing import Generator
+from pathlib import Path
+from playwright.sync_api import Playwright, BrowserContext
+import pytest
+
+
+@pytest.fixture()
+def context(playwright: Playwright) -> Generator[BrowserContext, None, None]:
+    path_to_extension = Path(__file__).parent.joinpath("my-extension")
+    context = playwright.chromium.launch_persistent_context(
+        "",
+        headless=False,
+        args=[
+            f"--disable-extensions-except={path_to_extension}",
+            f"--load-extension={path_to_extension}",
+        ],
+    )
+    yield context
+    context.close()
+
+
+@pytest.fixture()
+def extension_id(context) -> Generator[str, None, None]:
+    # for manifest v2:
+    # background = context.background_pages[0]
+    # if not background:
+    #     background = context.wait_for_event("backgroundpage")
+
+    # for manifest v3:
+    background = context.service_workers[0]
+    if not background:
+        background = context.wait_for_event("serviceworker")
+
+    extension_id = background.url.split("/")[2]
+    yield extension_id
+
+```
+
+```py
+# test_foo.py
+from playwright.sync_api import expect, Page
+
+
+def test_example_test(page: Page) -> None:
+    page.goto("https://example.com")
+    expect(page.locator("body")).to_contain_text("Changed by my-extension")
+
+
+def test_popup_page(page: Page, extension_id: str) -> None:
+    page.goto(f"chrome-extension://{extension_id}/popup.html")
+    expect(page.locator("body")).to_have_text("my-extension popup")
+```