[PR #11335/5b5eb8bc backport][3.12] small speedup by avoiding redundant IOBasePayload.size calls (#11338)

**This is a backport of PR #11335 as merged into master
(5b5eb8b).**

Co-authored-by: Cycloctane <[email protected]>

Author: patchback[bot]

aiohttp/multipart.py

@@ -972,14 +972,15 @@ def size(self) -> Optional[int]:
         """Size of the payload."""
         total = 0
         for part, encoding, te_encoding in self._parts:
-            if encoding or te_encoding or part.size is None:
+            part_size = part.size
+            if encoding or te_encoding or part_size is None:
                 return None
 
             total += int(
                 2
                 + len(self._boundary)
                 + 2
-                + part.size  # b'--'+self._boundary+b'\r\n'
+                + part_size  # b'--'+self._boundary+b'\r\n'
                 + len(part._binary_headers)
                 + 2  # b'\r\n'
             )

aiohttp/web_response.py

@@ -791,8 +791,8 @@ async def _start(self, request: "BaseRequest") -> AbstractStreamWriter:
                 del self._headers[hdrs.CONTENT_LENGTH]
         elif not self._chunked:
             if isinstance(self._body, Payload):
-                if self._body.size is not None:
-                    self._headers[hdrs.CONTENT_LENGTH] = str(self._body.size)
+                if (size := self._body.size) is not None:
+                    self._headers[hdrs.CONTENT_LENGTH] = str(size)
             else:
                 body_len = len(self._body) if self._body else "0"
                 # https://www.rfc-editor.org/rfc/rfc9110.html#section-8.6-7