gh-143930: Tweak the exception message and increase test coverage

Lib/test/test_webbrowser.py

@@ -57,6 +57,14 @@ def _test(self, meth, *, args=[URL], kw={}, options, arguments):
             popen_args.pop(popen_args.index(option))
         self.assertEqual(popen_args, arguments)
 
+    def test_reject_dash_prefixes(self):
+        browser = self.browser_class(name=CMD_NAME)
+        with self.assertRaisesRegex(
+            ValueError,
+            r"^Invalid URL \(leading dash disallowed\): '--key=val http.*'$"
+        ):
+            browser.open(f"--key=val {URL}")
+
 
 class GenericBrowserCommandTest(CommandTestMixin, unittest.TestCase):
 
@@ -67,11 +75,6 @@ def test_open(self):
                    options=[],
                    arguments=[URL])
 
-    def test_reject_dash_prefixes(self):
-        browser = self.browser_class(name=CMD_NAME)
-        with self.assertRaises(ValueError):
-            browser.open(f"--key=val {URL}")
-
 
 class BackgroundBrowserCommandTest(CommandTestMixin, unittest.TestCase):
 
@@ -326,7 +329,6 @@ def close(self):
 @unittest.skipUnless(sys.platform == "darwin", "macOS specific test")
 @requires_subprocess()
 class MacOSXOSAScriptTest(unittest.TestCase):
-
     def setUp(self):
         # Ensure that 'BROWSER' is not set to 'open' or something else.
         # See: https://github.com/python/cpython/issues/131254.
@@ -376,6 +378,13 @@ def test_explicit_browser(self):
         self.assertIn('tell application "safari"', script)
         self.assertIn('open location "https://python.org"', script)
 
+    def test_reject_dash_prefixes(self):
+        with self.assertRaisesRegex(
+            ValueError,
+            r"^Invalid URL \(leading dash disallowed\): '--key=val http.*'$"
+        ):
+            self.browser.open(f"--key=val {URL}")
+
 
 class BrowserRegistrationTest(unittest.TestCase):
 

Lib/webbrowser.py

@@ -167,7 +167,7 @@ def open_new_tab(self, url):
     def _check_url(url):
         """Ensures that the URL is safe to pass to subprocesses as a parameter"""
         if url and url.lstrip().startswith("-"):
-            raise ValueError(f"Invalid URL: {url}")
+            raise ValueError(f"Invalid URL (leading dash disallowed): {url!r}")
 
 
 class GenericBrowser(BaseBrowser):

Misc/NEWS.d/next/Security/2026-01-16-12-04-49.gh-issue-143930.zYC5x3.rst

@@ -1 +1 @@
-Reject leading dashes in URLs passed to :func:`webbrowser.open`
+Reject leading dashes in URLs passed to :func:`webbrowser.open`.