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gh-155389: Return bytes from _pyio.BytesIO.peek() #155390

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gh-155389: Return bytes from _pyio.BytesIO.peek() #155390

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gh-155389: Return bytes from _pyio.BytesIO.peek()
fedonman Aug 8, 2026
deeeb33
Keep only the slicing under the lock, and test a float size
fedonman Aug 8, 2026
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14 changes: 12 additions & 2 deletions Lib/_pyio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1003,9 +1003,19 @@ def tell(self):
def peek(self, size=0):
if self.closed:
raise ValueError("peek on closed file")
try:
size_index = size.__index__

@cmaloney cmaloney Aug 8, 2026

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that the size is ignored previously is actually intentional, that's the behavior in the C as well:

cpython/Modules/_io/bufferedio.c

Lines 951 to 982 in 998b890

/*[clinic input]
@critical_section
_io._Buffered.peek
size: Py_ssize_t = 0
/
[clinic start generated code]*/
static PyObject *
_io__Buffered_peek_impl(buffered *self, Py_ssize_t size)
/*[clinic end generated code: output=ba7a097ca230102b input=56733376f926d982]*/
{
PyObject *res = NULL;
CHECK_INITIALIZED(self)
CHECK_CLOSED(self, "peek of closed file")
if (!ENTER_BUFFERED(self))
return NULL;
if (self->writable) {
res = buffered_flush_and_rewind_unlocked(self);
if (res == NULL)
goto end;
Py_CLEAR(res);
}
res = _bufferedreader_peek_unlocked(self);
end:
LEAVE_BUFFERED(self)
return res;
}

@fedonman fedonman Aug 8, 2026

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I think that one is _io._Buffered.peek (BufferedReader), which does ignore size. BytesIO.peek is in bytesio.c and uses it: size < 1 becomes DEFAULT_BUFFER_SIZE, then it clamps to what is left in the buffer.

io.BytesIO.peek(2)      -> b'ab'
BufferedReader.peek(2)  -> b'bcdef'

So the Python fallback should honour it too, which is what the __index__ coercion is for.

except AttributeError:
raise TypeError(f"{size!r} is not an integer")
else:
size = size_index()

if size < 1:
return self._buffer[self._pos:self._pos + io.DEFAULT_BUFFER_SIZE]
return self._buffer[self._pos:self._pos + size]
size = io.DEFAULT_BUFFER_SIZE

with self._lock:
b = self._buffer[self._pos:self._pos + size]
return b.take_bytes()

def truncate(self, pos=None):
if self.closed:
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5 changes: 5 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 @@ -570,6 +570,11 @@ def test_peek(self):
buf = self.buftype("1234567890")
with self.ioclass(buf) as memio:
self.assertEqual(memio.tell(), 0)
# bytearray(b'1') == b'1', so the type has to be asserted separately.
self.assertIsInstance(memio.peek(), bytes)
self.assertIsInstance(memio.peek(1), bytes)
self.assertEqual(memio.peek(IntLike(3)), buf[:3])
self.assertRaises(TypeError, memio.peek, 1.5)
self.assertEqual(memio.peek(1), buf[:1])
self.assertEqual(memio.peek(1), buf[:1])
self.assertEqual(memio.peek(), buf)
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