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gh-159739: Handle overlapping buffers in BytesIO.readinto #159777

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gh-159739: Handle overlapping buffers in BytesIO.readinto #159777

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6 changes: 6 additions & 0 deletions Lib/test/test_io/test_memoryio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -962,6 +962,12 @@ class CBytesIOTest(PyBytesIOTest):
ioclass = io.BytesIO
UnsupportedOperation = io.UnsupportedOperation

def test_readinto_overlapping_buffer(self):
with self.ioclass(b'abcd') as stream:
with stream.getbuffer()[1:] as target:
self.assertEqual(stream.readinto(target), 3)
self.assertEqual(stream.getvalue(), b'aabc')

def test_getstate(self):
memio = self.ioclass()
state = memio.__getstate__()
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-10-19-20-38-17.gh-issue-159739.a81270.rst
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@@ -0,0 +1 @@
Fix :class:`io.BytesIO` reads into overlapping views of the same buffer.
2 changes: 1 addition & 1 deletion Modules/_io/bytesio.c
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Expand Up @@ -714,7 +714,7 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)

assert(self->pos + len <= PY_SSIZE_T_MAX);
assert(len >= 0);
memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
memmove(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
self->pos += len;

return PyLong_FromSsize_t(len);
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