From 2fbe89872d66dabc16a8ca442ac83d4d427b6fa4 Mon Sep 17 00:00:00 2001
From: Cody Maloney <cmaloney@theoreticalchaos.com>
Date: Sun, 26 Jan 2025 10:03:08 -0800
Subject: [PATCH] gh-129005: Avoid copy in _pyio.FileIO.readinto

`os.read` allocated and filled a buffer by calling `read(2)`, than that
data was copied into the user provied buffer. Read directly into the
caller's buffer instead by using `os.readinto`.

`os.readinto` uses `PyObject_GetBuffer` to make sure the passed in
buffer is writeable and bytes-like, drop the manual check.
---
 Lib/_pyio.py                                        | 13 +++++++------
 .../2025-01-26-10-01-21.gh-issue-129005.ncpLvw.rst  |  1 +
 2 files changed, 8 insertions(+), 6 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-01-26-10-01-21.gh-issue-129005.ncpLvw.rst

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 14961c39d3541d..023478aa78c6a0 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1692,13 +1692,14 @@ def readall(self):
 
         return bytes(result)
 
-    def readinto(self, b):
+    def readinto(self, buffer):
         """Same as RawIOBase.readinto()."""
-        m = memoryview(b).cast('B')
-        data = self.read(len(m))
-        n = len(data)
-        m[:n] = data
-        return n
+        self._checkClosed()
+        self._checkReadable()
+        try:
+            return os.readinto(self._fd, buffer)
+        except BlockingIOError:
+            return None
 
     def write(self, b):
         """Write bytes b to file, return number written.
diff --git a/Misc/NEWS.d/next/Library/2025-01-26-10-01-21.gh-issue-129005.ncpLvw.rst b/Misc/NEWS.d/next/Library/2025-01-26-10-01-21.gh-issue-129005.ncpLvw.rst
new file mode 100644
index 00000000000000..a825e9d244d525
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-01-26-10-01-21.gh-issue-129005.ncpLvw.rst
@@ -0,0 +1 @@
+Optimize ``_pyio.FileIO.readinto`` by avoiding unnecessary objects and copies using :func:`os.readinto`.