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

docs/api/cookies.rst

@@ -39,15 +39,13 @@ The :py:attr:`~.Request.cookies` attribute is a regular
 Setting Cookies
 ~~~~~~~~~~~~~~~
 
-Setting cookies on a response is done via :py:meth:`~.Response.set_cookie`.
-
-The :py:meth:`~.Response.set_cookie` method should be used instead of
-:py:meth:`~.Response.set_header` or :py:meth:`~.Response.append_header`.
-With :py:meth:`~.Response.set_header` you cannot set multiple headers
-with the same name (which is how multiple cookies are sent to the client).
-Furthermore, :py:meth:`~.Response.append_header` appends multiple values
-to the same header field in a way that is not compatible with the special
-format required by the `Set-Cookie` header.
+Setting cookies on a response may be done either via
+:py:meth:`~.Response.set_cookie` or :py:meth:`~.Response.append_header`.
+
+One of these methods should be used instead of
+:py:meth:`~.Response.set_header`. With :py:meth:`~.Response.set_header` you
+cannot set multiple headers with the same name (which is how multiple cookies
+are sent to the client).
 
 Simple example:
 

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()
 
@@ -551,7 +554,7 @@ def append_header(self, name, value):
         Note:
             While this method can be used to efficiently append raw
             Set-Cookie headers to the response, you may find
-            :py:meth:`~.set_cookie` more convenient.
+            :py:meth:`~.set_cookie` to be more convenient.
 
         Args:
             name (str): Header name (case-insensitive). The restrictions
@@ -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