new list item valid test completed, even if it doesn't yet pass

src/functional_tests/base.py

@@ -31,3 +31,12 @@ class FunctionalTest(StaticLiveServerTestCase):
                     raise
                 time.sleep(0.5)
                 
+    def wait_for(self, fn):
+        start_time = time.time()
+        while True:
+            try:
+                return fn()
+            except (AssertionError, WebDriverException):
+                if time.time() - start_time > MAX_WAIT:
+                    raise
+                time.sleep(0.5)

src/functional_tests/test_list_item_validation.py

@@ -5,4 +5,31 @@ from .base import FunctionalTest
 
 class ItemValidationTest(FunctionalTest):
     def test_cannot_add_empty_list_items(self):
-        self.fail("write me!")
+        self.browser.get(self.live_server_url)
+        self.browser.find_element(By.ID, "id-new-item").send_keys(Keys.ENTER)
+
+        self.wait_for(
+            lambda: self.assertEqual(
+                self.browser.find_element(By.CSS_SELECTOR, ".invalid-feedback").text,
+                "You can't have an empty list item",
+            )
+        )
+
+        self.browser.find_element(By.ID, "id-new-item").send_keys("Purchase milk")
+        self.browser.find_element(By.ID, "id-new-item").send_keys(Keys.ENTER)
+        self.wait_for_row_in_list_table("1. Purchase milk")
+
+        self.browser.find_element(By.ID, "id-new-item").send_keys(Keys.ENTER)
+
+        self.wait_for(
+            lambda: self.assertEqual(
+                self.browser.find_element(By.CSS_SELECTOR, ".invalid-feedback").text,
+                "You can't have an empty list item",
+            )
+        )
+
+        self.browser.find_element(By.ID, 'id-new-item').send_keys("Make tea")
+        self.browser.find_element(By.ID, "id-new-item").send_keys(Keys.ENTER)
+        self.wait_for_row_in_list_table("2. Make tea")
+
+        self.fail("finish the fight!")