gh-128540: lookup default webbrowser on Windows

Lib/test/test_webbrowser.py

@@ -301,6 +301,44 @@ def test_open_new_tab(self):
         self._test('open_new_tab')
 
 
+@unittest.skipUnless(sys.platform[:3] == "win", "Windows test")
+class WindowsDefaultTest(unittest.TestCase):
+    def setUp(self):
+        support.patch(self, os, "startfile", mock.Mock())
+        self.browser = webbrowser.WindowsDefault()
+        support.patch(self, self.browser, "_open_default_browser", mock.Mock())
+
+    def test_default(self):
+        browser = webbrowser.get()
+        self.assertIsInstance(browser, webbrowser.WindowsDefault)
+
+    def test_open_startfile(self):
+        url = "https://python.org"
+        self.browser.open(url)
+        self.browser._open_default_browser.assert_not_called()
+        os.startfile.assert_called_with(url)
+
+    def test_open_browser_lookup(self):
+        url = "file://python.org"
+        self.browser.open(url)
+        self.browser._open_default_browser.assert_called_with(url)
+        os.startfile.assert_not_called()
+
+    def test_open_browser_lookup_fails(self):
+        url = "file://python.org"
+        self.browser._open_default_browser.return_value = False
+        self.browser.open(url)
+        self.browser._open_default_browser.assert_called_with(url)
+        os.startfile.assert_called_with(url)
+
+    def test_open_browser_lookup_error(self):
+        url = "file://python.org"
+        self.browser._open_default_browser.side_effect = OSError('registry failed...')
+        self.browser.open(url)
+        self.browser._open_default_browser.assert_called_with(url)
+        os.startfile.assert_called_with(url)
+
+
 class BrowserRegistrationTest(unittest.TestCase):
 
     def setUp(self):

Lib/webbrowser.py

@@ -573,8 +573,61 @@ def register_standard_browsers():
 
 if sys.platform[:3] == "win":
     class WindowsDefault(BaseBrowser):
+        def _open_default_browser(self, url):
+            """Open a URL with the default browser
+
+            launches the web browser no matter what `url` is, unlike startfile.
+
+            Raises OSError if registry lookups fail.
+            Returns False if URL not opened.
+            """
+            try:
+                import winreg
+            except ImportError:
+                return False
+            # lookup progId for https URLs
+            # e.g. 'FirefoxURL-abc123'
+            with winreg.OpenKey(
+                winreg.HKEY_CURRENT_USER,
+                r"Software\Microsoft\Windows\Shell\Associations\UrlAssociations\https\UserChoice",
+            ) as key:
+                browser_id = winreg.QueryValueEx(key, "ProgId")[0]
+            # lookup launch command-line
+            # e.g. '"C:\\Program Files\\Mozilla Firefox\\firefox.exe" -osint -url "%1"'
+            with winreg.OpenKey(
+                winreg.HKEY_CLASSES_ROOT,
+                rf"{browser_id}\shell\open\command",
+            ) as key:
+                browser_cmd = winreg.QueryValueEx(key, "")[0]
+
+            # build command-line
+            if "%1" not in browser_cmd:
+                # Command is missing '%1' placeholder,
+                # so we don't know how to build the command to open a file
+                # would append be safe in this case?
+                return False
+
+            # the rest copied from BackgroundBrowser
+            cmdline = [arg.replace("%1", url) for arg in shlex.split(browser_cmd)]
+            try:
+                p = subprocess.Popen(cmdline)
+                return p.poll() is None
+            except OSError:
+                return False
+
         def open(self, url, new=0, autoraise=True):
             sys.audit("webbrowser.open", url)
+            proto, _sep, _rest = url.partition(":")
+            if _sep and proto.lower() not in {"http", "https"}:
+                # need to lookup browser if it's not a web URL
+                try:
+                    opened = self._open_default_browser(url)
+                except OSError:
+                    # failed to lookup registry items
+                    opened = False
+                if opened:
+                    return opened
+
             try:
                 os.startfile(url)
             except OSError:

Misc/NEWS.d/next/Windows/2025-02-25-11-38-45.gh-issue-128540.nTM0bU.rst

@@ -0,0 +1,2 @@
+Ensure web browser is launched by :func:`webbrowser.open` on Windows, even
+for ``file://`` URLs.