gh-128540: lookup default webbrowser on macOS (#130535)

Ensure web browser is launched by `webbrowser.open` on macOS, even for `file://` URLs.

Lib/test/test_webbrowser.py

@@ -1,3 +1,4 @@
+import io
 import os
 import re
 import shlex
@@ -5,6 +6,7 @@ import subprocess
 import sys
 import unittest
 import webbrowser
+from functools import partial
 from test import support
 from test.support import import_helper
 from test.support import is_apple_mobile
@@ -301,6 +303,69 @@ class IOSBrowserTest(unittest.TestCase):
         self._test('open_new_tab')
 
 
+class MockPopenPipe:
+    def __init__(self, cmd, mode):
+        self.cmd = cmd
+        self.mode = mode
+        self.pipe = io.StringIO()
+        self._closed = False
+
+    def write(self, buf):
+        self.pipe.write(buf)
+
+    def close(self):
+        self._closed = True
+        return None
+
+
+@unittest.skipUnless(sys.platform == "darwin", "macOS specific test")
+@requires_subprocess()
+class MacOSXOSAScriptTest(unittest.TestCase):
+    def setUp(self):
+        support.patch(self, os, "popen", self.mock_popen)
+        self.browser = webbrowser.MacOSXOSAScript("default")
+
+    def mock_popen(self, cmd, mode):
+        self.popen_pipe = MockPopenPipe(cmd, mode)
+        return self.popen_pipe
+
+    def test_default(self):
+        browser = webbrowser.get()
+        assert isinstance(browser, webbrowser.MacOSXOSAScript)
+        self.assertEqual(browser.name, "default")
+
+    def test_default_open(self):
+        url = "https://python.org"
+        self.browser.open(url)
+        self.assertTrue(self.popen_pipe._closed)
+        self.assertEqual(self.popen_pipe.cmd, "osascript")
+        script = self.popen_pipe.pipe.getvalue()
+        self.assertEqual(script.strip(), f'open location "{url}"')
+
+    def test_url_quote(self):
+        self.browser.open('https://python.org/"quote"')
+        script = self.popen_pipe.pipe.getvalue()
+        self.assertEqual(
+            script.strip(), 'open location "https://python.org/%22quote%22"'
+        )
+
+    def test_default_browser_lookup(self):
+        url = "file:///tmp/some-file.html"
+        self.browser.open(url)
+        script = self.popen_pipe.pipe.getvalue()
+        # doesn't actually test the browser lookup works,
+        # just that the branch is taken
+        self.assertIn("URLForApplicationToOpenURL", script)
+        self.assertIn(f'open location "{url}"', script)
+
+    def test_explicit_browser(self):
+        browser = webbrowser.MacOSXOSAScript("safari")
+        browser.open("https://python.org")
+        script = self.popen_pipe.pipe.getvalue()
+        self.assertIn('tell application "safari"', script)
+        self.assertIn('open location "https://python.org"', script)
+
+
 class BrowserRegistrationTest(unittest.TestCase):
 
     def setUp(self):

Lib/webbrowser.py

@@ -597,7 +597,32 @@ if sys.platform == 'darwin':
             sys.audit("webbrowser.open", url)
             url = url.replace('"', '%22')
             if self.name == 'default':
-                script = f'open location "{url}"'  # opens in default browser
+                proto, _sep, _rest = url.partition(":")
+                if _sep and proto.lower() in {"http", "https"}:
+                    # default web URL, don't need to lookup browser
+                    script = f'open location "{url}"'
+                else:
+                    # if not a web URL, need to lookup default browser to ensure a browser is launched
+                    # this should always work, but is overkill to lookup http handler
+                    # before launching http
+                    script = f"""
+                        use framework "AppKit"
+                        use AppleScript version "2.4"
+                        use scripting additions
+
+                        property NSWorkspace : a reference to current application's NSWorkspace
+                        property NSURL : a reference to current application's NSURL
+
+                        set http_url to NSURL's URLWithString:"https://python.org"
+                        set browser_url to (NSWorkspace's sharedWorkspace)'s ¬
+                            URLForApplicationToOpenURL:http_url
+                        set app_path to browser_url's relativePath as text -- NSURL to absolute path '/Applications/Safari.app'
+
+                        tell application app_path
+                            activate
+                            open location "{url}"
+                        end tell
+                    """
             else:
                 script = f'''
                    tell application "{self.name}"

Misc/NEWS.d/next/macOS/2025-02-25-10-25-27.gh-issue-128540.QDz3OL.rst

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