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 76a27910da4d5f375508881b565373dc523e9bfe..755e025877089136af068a8edafc1070ed08b3dd 100644 (file)
--- 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 (file)
index 0000000..48ee571
--- /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.