From: Cody Maloney <cmaloney@users.noreply.github.com>
Date: Thu, 30 Jan 2025 11:23:25 +0000 (-0800)
Subject: gh-129005: _pyio.BufferedIO remove copy on readall (#129454)
X-Git-Tag: v3.14.0a5~170
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=e1c4ba928852eac0b0e0bded1c314e3e36975286;p=thirdparty%2FPython%2Fcpython.git

gh-129005: _pyio.BufferedIO remove copy on readall (#129454)

Slicing buf and appending chunk would always result in a copy. Commonly
in a readall() there is no already read data in buf, and the amount of
data read may be large, so the copy is expensive.
---

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 76a27910da4d..755e02587708 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1062,6 +1062,9 @@ class BufferedReader(_BufferedIOMixin):
                 if chunk is None:
                     return buf[pos:] or None
                 else:
+                    # Avoid slice + copy if there is no data in buf
+                    if not buf:
+                        return chunk
                     return buf[pos:] + chunk
             chunks = [buf[pos:]]  # Strip the consumed bytes.
             current_size = 0
diff --git a/Misc/NEWS.d/next/Library/2025-01-29-00-00-01.gh-issue-129005.aV_3O8.rst b/Misc/NEWS.d/next/Library/2025-01-29-00-00-01.gh-issue-129005.aV_3O8.rst
new file mode 100644
index 000000000000..48ee57109be2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-01-29-00-00-01.gh-issue-129005.aV_3O8.rst
@@ -0,0 +1,2 @@
+:mod:`!_pyio`: Remove an unnecessary copy when ``_pyio.BufferedReader.read()``
+is called to read all data from a file and has no data already in buffer.