gh-129005: Remove copy in _pyio.FileIO.readall()

This aligns the memory usage between _pyio and _io. Both now use the
same amount of memory when reading a file.

Author: cmaloney

Lib/_pyio.py

@@ -1702,7 +1702,7 @@ def readall(self):
             bytes_read += n
 
         del result[bytes_read:]
-        return bytes(result)
+        return result
 
     def readinto(self, buffer):
         """Same as RawIOBase.readinto()."""

Lib/test/test_largefile.py

@@ -56,9 +56,7 @@ class TestFileMethods(LargeFileTest):
     (i.e. > 2 GiB) files.
     """
 
-    # _pyio.FileIO.readall() uses a temporary bytearray then casted to bytes,
-    # so memuse=2 is needed
-    @bigmemtest(size=size, memuse=2, dry_run=False)
+    @bigmemtest(size=size, memuse=1, dry_run=False)
     def test_large_read(self, _size):
         # bpo-24658: Test that a read greater than 2GB does not fail.
         with self.open(TESTFN, "rb") as f:
@@ -154,7 +152,7 @@ def test_seekable(self):
                 f.seek(pos)
                 self.assertTrue(f.seekable())
 
-    @bigmemtest(size=size, memuse=2, dry_run=False)
+    @bigmemtest(size=size, memuse=1, dry_run=False)
     def test_seek_readall(self, _size):
         # Seek which doesn't change position should readall successfully.
         with self.open(TESTFN, 'rb') as f:

Misc/NEWS.d/next/Library/2025-01-28-21-51-50.gh-issue-129005.OIdODu.rst

@@ -0,0 +1,2 @@
+:mod:`!_pyio`: Return bytearray from ``_pyio.FileIO.readall()`` rather than
+copying into a bytes. Memory usage now matches ``_io.FileIO.readall()``