gh-90533: Implement BytesIO.peek() (#150917) · python/cpython@72cad14 · GitHub
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marcelmblurb-it[bot]erlend-aaslandvstinneremmatyping
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gh-90533: Implement BytesIO.peek() (#150917)
Add io.BytesIO.peek() method to read without advancing position. Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com> Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com> Co-authored-by: Victor Stinner <vstinner@python.org> Co-authored-by: Emma Smith <emma@emmatyping.dev> Co-authored-by: Stan Ulbrych <stan@python.org> Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>
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‎Doc/library/io.rst‎

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‎Doc/whatsnew/3.16.rst‎

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@@ -163,6 +163,12 @@ gzip
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which is passed on to the constructor of the :class:`~gzip.GzipFile` class.
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(Contributed by Marin Misur in :gh:`91372`.)
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io
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--
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* Add :meth:`io.BytesIO.peek` method to read without advancing position.
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(Contributed by Marcel Martin in :gh:`90533`.)
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logging
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-------

‎Lib/_pyio.py‎

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@@ -1000,6 +1000,13 @@ def tell(self):
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raise ValueError("tell on closed file")
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return self._pos
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def peek(self, size=0):
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if self.closed:
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raise ValueError("peek on closed file")
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if size < 1:
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return self._buffer[self._pos:self._pos + io.DEFAULT_BUFFER_SIZE]
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return self._buffer[self._pos:self._pos + size]
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def truncate(self, pos=None):
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if self.closed:
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raise ValueError("truncate on closed file")

‎Lib/test/test_free_threading/test_io.py‎

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@@ -67,6 +67,10 @@ def readinto(barrier, b, into, *ignore):
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barrier.wait()
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b.readinto(into)
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def peek(barrier, b, *ignore):
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barrier.wait()
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b.peek()
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def close(barrier, b, *ignore):
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barrier.wait()
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b.close()
@@ -103,6 +107,7 @@ def sizeof(barrier, b, *ignore):
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self.check([truncate] + [readline] * 10, self.ioclass(b'0\n'*20480))
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self.check([truncate] + [readlines] * 10, self.ioclass(b'0\n'*20480))
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self.check([truncate] + [readinto] * 10, self.ioclass(b'0\n'*204800), bytearray(b'0\n'*204800))
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self.check([truncate] + [peek] * 10, self.ioclass(b'0\n'*204800))
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self.check([close] + [write] * 10, self.ioclass())
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self.check([truncate] + [getvalue] * 10, self.ioclass(b'0\n'*204800))
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self.check([truncate] + [getbuffer] * 10, self.ioclass(b'0\n'*204800))

‎Lib/test/test_io/test_memoryio.py‎

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@@ -566,6 +566,77 @@ def test_issue141311(self):
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buf = bytearray(2)
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self.assertEqual(0, memio.readinto(buf))
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def test_peek(self):
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buf = self.buftype("1234567890")
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with self.ioclass(buf) as memio:
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self.assertEqual(memio.tell(), 0)
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self.assertEqual(memio.peek(1), buf[:1])
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self.assertEqual(memio.peek(1), buf[:1])
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self.assertEqual(memio.peek(), buf)
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self.assertEqual(memio.peek(3), buf[:3])
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self.assertEqual(memio.peek(5), buf[:5])
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self.assertEqual(memio.peek(0), buf)
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self.assertEqual(memio.peek(len(buf) + 100), buf)
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self.assertEqual(memio.peek(-1), buf)
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self.assertEqual(memio.tell(), 0)
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memio.read(1)
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self.assertEqual(memio.tell(), 1)
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self.assertEqual(memio.peek(1), buf[1:2])
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self.assertEqual(memio.peek(), buf[1:])
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self.assertEqual(memio.peek(3), buf[1:4])
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self.assertEqual(memio.peek(5), buf[1:6])
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self.assertEqual(memio.peek(0), buf[1:])
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self.assertEqual(memio.peek(len(buf) + 100), buf[1:])
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self.assertEqual(memio.peek(-1), buf[1:])
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self.assertEqual(memio.tell(), 1)
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memio.read()
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self.assertEqual(memio.tell(), len(buf))
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self.assertEqual(memio.peek(1), self.EOF)
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self.assertEqual(memio.peek(3), self.EOF)
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self.assertEqual(memio.peek(5), self.EOF)
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self.assertEqual(memio.peek(0), b"")
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self.assertEqual(memio.tell(), len(buf))
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# Peeking works after writing
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abc = self.buftype("abc")
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memio.write(abc)
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self.assertEqual(memio.peek(), self.EOF)
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memio.seek(len(buf))
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self.assertEqual(memio.peek(), abc)
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self.assertEqual(memio.peek(-1), abc)
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self.assertEqual(memio.peek(len(abc) + 100), abc)
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self.assertEqual(memio.tell(), len(buf))
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with self.ioclass(buf) as memio:
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memio.seek(len(buf))
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self.assertEqual(memio.peek(), self.EOF)
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# Length greater than DEFAULT_BUFFER_SIZE
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buf = self.buftype("1234567890" * io.DEFAULT_BUFFER_SIZE)
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with self.ioclass(buf) as memio:
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self.assertEqual(memio.peek(), buf[:io.DEFAULT_BUFFER_SIZE])
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self.assertEqual(memio.peek(0), buf[:io.DEFAULT_BUFFER_SIZE])
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self.assertEqual(memio.peek(-1), buf[:io.DEFAULT_BUFFER_SIZE])
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self.assertEqual(memio.peek(io.DEFAULT_BUFFER_SIZE + 100),
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buf[:io.DEFAULT_BUFFER_SIZE + 100])
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self.assertEqual(memio.peek(io.DEFAULT_BUFFER_SIZE * 100), buf)
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# Current position beyond buffer end
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with self.ioclass(buf) as memio:
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memio.seek(len(buf) + 100)
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self.assertEqual(memio.peek(), self.EOF)
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with self.ioclass(buf) as memio:
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memio.read()
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memio.truncate(0)
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self.assertEqual(memio.tell(), len(buf))
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self.assertEqual(memio.peek(), self.EOF)
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# Peek after close raises
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self.assertRaises(ValueError, memio.peek)
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def test_unicode(self):
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memio = self.ioclass()
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‎Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst‎

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Add :meth:`io.BytesIO.peek` method to read without advancing position.

‎Modules/_io/bytesio.c‎

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‎Modules/_io/clinic/bytesio.c.h‎

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