perf(respone): Lazy instantiation of the extra headers list object

falcon/response.py

@@ -168,7 +168,10 @@ def __init__(self, options=None):
         # NOTE(kgriffs): Collection of additional headers as a list of raw
         #   tuples, to use in cases where we need more control over setting
         #   headers and duplicates are allowable or even necessary.
-        self._extra_headers = []
+        #
+        # PERF(kgriffs): Save some CPU cycles and a few bytes of RAM by
+        #   only instantiating the list object later on IFF it is needed.
+        self._extra_headers = None
 
         self.options = options if options else ResponseOptions()
 
@@ -574,7 +577,10 @@ def append_header(self, name, value):
         name = name.lower()
 
         if name == 'set-cookie':
-            self._extra_headers.append((name, value))
+            if not self._extra_headers:
+                self._extra_headers = [(name, value)]
+            else:
+                self._extra_headers.append((name, value))
         else:
             if name in self._headers:
                 value = self._headers[name] + ', ' + value