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gh-139871: Optimize small takes in bytearray.take_bytes (GH-141741)

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When less than half the buffer is taken just copy that small part out
rather than doing a big alloc + memmove + big shrink.
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Cody Maloney 2025-11-19 23:49:05 -08:00 • committed by GitHub
parent a35c683da5
commit e265ce8a56
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2 changed files with 39 additions and 1 deletions
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26
Lib/test/test_bytes.py
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@ -1524,6 +1524,32 @@ def test_take_bytes(self):
self.assertRaises(BufferError, ba.take_bytes)
self.assertEqual(ba.take_bytes(), b'abc')
@support.cpython_only # tests an implementation detail
def test_take_bytes_optimization(self):
# Validate optimization around taking lots of little chunks out of a
# much bigger buffer. Save work by only copying a little rather than
# moving a lot.
ba = bytearray(b'abcdef' + b'0' * 1000)
start_alloc = ba.__alloc__()
# Take two bytes at a time, checking alloc doesn't change.
self.assertEqual(ba.take_bytes(2), b'ab')
self.assertEqual(ba.__alloc__(), start_alloc)
self.assertEqual(len(ba), 4 + 1000)
self.assertEqual(ba.take_bytes(2), b'cd')
self.assertEqual(ba.__alloc__(), start_alloc)
self.assertEqual(len(ba), 2 + 1000)
self.assertEqual(ba.take_bytes(2), b'ef')
self.assertEqual(ba.__alloc__(), start_alloc)
self.assertEqual(len(ba), 0 + 1000)
self.assertEqual(ba.__alloc__(), start_alloc)
# Take over half, alloc shrinks to exact size.
self.assertEqual(ba.take_bytes(501), b'0' * 501)
self.assertEqual(len(ba), 499)
bytes_header_size = sys.getsizeof(b'')
self.assertEqual(ba.__alloc__(), 499 + bytes_header_size)
def test_setitem(self):
def setitem_as_mapping(b, i, val):
b[i] = val