Convert to fstr, move check

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: StanFromIreland

Lib/http/cookies.py

@@ -338,7 +338,7 @@ def update(self, values):
             if key not in self._reserved:
                 raise CookieError("Invalid attribute %r" % (key,))
             if _has_control_character(key, val):
-                raise CookieError("Control characters are not allowed in cookies %r %r" % (key, val))
+                raise CookieError(f"Control characters are not allowed in cookies {key!r} {val!r}")
             data[key] = val
         dict.update(self, data)
 
@@ -385,13 +385,16 @@ def __repr__(self):
 
     def js_output(self, attrs=None):
         # Print javascript
+        output_string = self.OutputString(attrs)
+        if _has_control_character(output_string):
+            raise CookieError("Control characters are not allowed in cookies")
         return """
         <script type="text/javascript">
         <!-- begin hiding
         document.cookie = \"%s\";
         // end hiding -->
         </script>
-        """ % (self.OutputString(attrs).replace('"', r'\"'))
+        """ % (output_string.replace('"', r'\"'))
 
     def OutputString(self, attrs=None):
         # Build up our result
@@ -530,8 +533,6 @@ def js_output(self, attrs=None):
         result = []
         items = sorted(self.items())
         for key, value in items:
-            if _has_control_character(value.OutputString(attrs)):
-                raise CookieError("Control characters are not allowed in cookies")
             result.append(value.js_output(attrs))
         return _nulljoin(result)