gh-160782: fix fish quote-escaping gap

Lib/test/test_venv.py

@@ -846,6 +846,18 @@ def test_fish_activate_shadowed_builtins(self):
         self.assertNotIn('LEAK', text)
         self.assertIn('OLDSTATUS=7', text)
 
+    def test_fish_quoting(self):
+        """
+        Test that the template strings are quoted properly (fish)
+        """
+        builder = venv.EnvBuilder()
+        context = builder.ensure_directories(self.env_dir)
+        context.script_path = 'activate.fish'
+        context.prompt = r"x\'$(echo PWNED)'\'"
+        self.assertEqual(
+            builder.replace_variables('__VENV_PROMPT__', context),
+            r"'x\\\'$(echo PWNED)\'\\\''")
+
     # gh-124651: test quoted strings on Windows
     @unittest.skipUnless(os.name == 'nt', 'only relevant on Windows')
     def test_special_chars_windows(self):

Lib/venv/__init__.py

@@ -521,13 +521,20 @@ def quote_ps1(s):
         def quote_bat(s):
             return s
 
+        def quote_fish(s):
+            # gh-160782: fish reads \\ and \' as escapes inside single quotes
+            s = s.replace('\\', '\\\\').replace("'", "\\'")
+            return f"'{s}'"
+
         # gh-124651: need to quote the template strings properly
         quote = shlex.quote
         script_path = context.script_path
         if script_path.endswith('.ps1'):
             quote = quote_ps1
         elif script_path.endswith('.bat'):
             quote = quote_bat
+        elif script_path.endswith('.fish'):
+            quote = quote_fish
         else:
             # fallbacks to POSIX shell compliant quote
             quote = shlex.quote

Misc/NEWS.d/next/Library/2026-10-29-23-25-31.gh-issue-160782.1ZwCOr.rst

@@ -0,0 +1,2 @@
+:mod:`venv`: a backslash before a quote in the prompt or the path no longer
+breaks ``activate.fish``.