diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index de5cab5aee649f..dfebccb5a9cb91 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -719,6 +719,9 @@ than raw I/O does.
The optional argument *initial_bytes* is a :term:`bytes-like object` that
contains initial data.
+ Methods may be used from multiple threads without external locking in
+ :term:`free threading` builds.
+
:class:`BytesIO` provides or overrides these methods in addition to those
from :class:`BufferedIOBase` and :class:`IOBase`:
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index fb2a6d049caab6..5db8ce9244b5ba 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -876,16 +876,28 @@ class BytesIO(BufferedIOBase):
_buffer = None
def __init__(self, initial_bytes=None):
+ # Use to keep self._buffer and self._pos consistent.
+ self._lock = Lock()
+
buf = bytearray()
if initial_bytes is not None:
buf += initial_bytes
- self._buffer = buf
- self._pos = 0
+
+ with self._lock:
+ self._buffer = buf
+ self._pos = 0
def __getstate__(self):
if self.closed:
raise ValueError("__getstate__ on closed file")
- return self.__dict__.copy()
+ with self._lock:
+ state = self.__dict__.copy()
+ del state['_lock']
+ return state
+
+ def __setstate__(self, state):
+ self.__dict__.update(state)
+ self._lock = Lock()
def getvalue(self):
"""Return the bytes value (contents) of the buffer
@@ -918,14 +930,16 @@ def read(self, size=-1):
raise TypeError(f"{size!r} is not an integer")
else:
size = size_index()
- if size < 0:
- size = len(self._buffer)
- if len(self._buffer) <= self._pos:
- return b""
- newpos = min(len(self._buffer), self._pos + size)
- b = self._buffer[self._pos : newpos]
- self._pos = newpos
- return bytes(b)
+
+ with self._lock:
+ if size < 0:
+ size = len(self._buffer)
+ if len(self._buffer) <= self._pos:
+ return b""
+ newpos = min(len(self._buffer), self._pos + size)
+ b = self._buffer[self._pos : newpos]
+ self._pos = newpos
+ return bytes(b)
def read1(self, size=-1):
"""This is the same as read.
@@ -941,12 +955,14 @@ def write(self, b):
n = view.nbytes # Size of any bytes-like object
if n == 0:
return 0
- pos = self._pos
- if pos > len(self._buffer):
- # Pad buffer to pos with null bytes.
- self._buffer.resize(pos)
- self._buffer[pos:pos + n] = b
- self._pos += n
+
+ with self._lock:
+ pos = self._pos
+ if pos > len(self._buffer):
+ # Pad buffer to pos with null bytes.
+ self._buffer.resize(pos)
+ self._buffer[pos:pos + n] = b
+ self._pos += n
return n
def seek(self, pos, whence=0):
@@ -963,9 +979,11 @@ def seek(self, pos, whence=0):
raise ValueError("negative seek position %r" % (pos,))
self._pos = pos
elif whence == 1:
- self._pos = max(0, self._pos + pos)
+ with self._lock:
+ self._pos = max(0, self._pos + pos)
elif whence == 2:
- self._pos = max(0, len(self._buffer) + pos)
+ with self._lock:
+ self._pos = max(0, len(self._buffer) + pos)
else:
raise ValueError("unsupported whence value")
return self._pos
@@ -978,18 +996,20 @@ def tell(self):
def truncate(self, pos=None):
if self.closed:
raise ValueError("truncate on closed file")
- if pos is None:
- pos = self._pos
- else:
- try:
- pos_index = pos.__index__
- except AttributeError:
- raise TypeError(f"{pos!r} is not an integer")
+
+ with self._lock:
+ if pos is None:
+ pos = self._pos
else:
- pos = pos_index()
- if pos < 0:
- raise ValueError("negative truncate position %r" % (pos,))
- del self._buffer[pos:]
+ try:
+ pos_index = pos.__index__
+ except AttributeError:
+ raise TypeError(f"{pos!r} is not an integer")
+ else:
+ pos = pos_index()
+ if pos < 0:
+ raise ValueError("negative truncate position %r" % (pos,))
+ del self._buffer[pos:]
return pos
def readable(self):
diff --git a/Lib/test/test_free_threading/test_io.py b/Lib/test/test_free_threading/test_io.py
index f9bec740ddff50..41d89e04da8716 100644
--- a/Lib/test/test_free_threading/test_io.py
+++ b/Lib/test/test_free_threading/test_io.py
@@ -1,12 +1,13 @@
+import io
+import _pyio as pyio
import threading
from unittest import TestCase
from test.support import threading_helper
from random import randint
-from io import BytesIO
from sys import getsizeof
-class TestBytesIO(TestCase):
+class ThreadSafetyMixin:
# Test pretty much everything that can break under free-threading.
# Non-deterministic, but at least one of these things will fail if
# BytesIO object is not free-thread safe.
@@ -90,20 +91,27 @@ def sizeof(barrier, b, *ignore):
barrier.wait()
getsizeof(b)
- self.check([write] * 10, BytesIO())
- self.check([writelines] * 10, BytesIO())
- self.check([write] * 10 + [truncate] * 10, BytesIO())
- self.check([truncate] + [read] * 10, BytesIO(b'0\n'*204800))
- self.check([truncate] + [read1] * 10, BytesIO(b'0\n'*204800))
- self.check([truncate] + [readline] * 10, BytesIO(b'0\n'*20480))
- self.check([truncate] + [readlines] * 10, BytesIO(b'0\n'*20480))
- self.check([truncate] + [readinto] * 10, BytesIO(b'0\n'*204800), bytearray(b'0\n'*204800))
- self.check([close] + [write] * 10, BytesIO())
- self.check([truncate] + [getvalue] * 10, BytesIO(b'0\n'*204800))
- self.check([truncate] + [getbuffer] * 10, BytesIO(b'0\n'*204800))
- self.check([truncate] + [iter] * 10, BytesIO(b'0\n'*20480))
- self.check([truncate] + [getstate] * 10, BytesIO(b'0\n'*204800))
- self.check([truncate] + [setstate] * 10, BytesIO(b'0\n'*204800), (b'123', 0, None))
- self.check([truncate] + [sizeof] * 10, BytesIO(b'0\n'*204800))
+ self.check([write] * 10, self.ioclass())
+ self.check([writelines] * 10, self.ioclass())
+ self.check([write] * 10 + [truncate] * 10, self.ioclass())
+ self.check([truncate] + [read] * 10, self.ioclass(b'0\n'*204800))
+ self.check([truncate] + [read1] * 10, self.ioclass(b'0\n'*204800))
+ self.check([truncate] + [readline] * 10, self.ioclass(b'0\n'*20480))
+ self.check([truncate] + [readlines] * 10, self.ioclass(b'0\n'*20480))
+ self.check([truncate] + [readinto] * 10, self.ioclass(b'0\n'*204800), bytearray(b'0\n'*204800))
+ self.check([close] + [write] * 10, self.ioclass())
+ self.check([truncate] + [getvalue] * 10, self.ioclass(b'0\n'*204800))
+ self.check([truncate] + [getbuffer] * 10, self.ioclass(b'0\n'*204800))
+ self.check([truncate] + [iter] * 10, self.ioclass(b'0\n'*20480))
+ self.check([truncate] + [getstate] * 10, self.ioclass(b'0\n'*204800))
+ state = self.ioclass(b'123').__getstate__()
+ self.check([truncate] + [setstate] * 10, self.ioclass(b'0\n'*204800), state)
+ self.check([truncate] + [sizeof] * 10, self.ioclass(b'0\n'*204800))
# no tests for seek or tell because they don't break anything
+
+class CBytesIOTest(ThreadSafetyMixin, TestCase):
+ ioclass = io.BytesIO
+
+class PyBytesIOTest(ThreadSafetyMixin, TestCase):
+ ioclass = pyio.BytesIO
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 0c921ffbc2576a..b487bcabf01ca4 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -9,6 +9,7 @@
# * test_univnewlines - tests universal newline support
# * test_largefile - tests operations on a file greater than 2**32 bytes
# (only enabled with -ulargefile)
+# * test_free_threading/test_io - tests thread safety of io objects
################################################################################
# ATTENTION TEST WRITERS!!!
diff --git a/Misc/NEWS.d/next/Library/2025-07-02-18-41-45.gh-issue-133982.7qqAn6.rst b/Misc/NEWS.d/next/Library/2025-07-02-18-41-45.gh-issue-133982.7qqAn6.rst
new file mode 100644
index 00000000000000..a2d0810cebeaa6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-07-02-18-41-45.gh-issue-133982.7qqAn6.rst
@@ -0,0 +1 @@
+Update Python implementation of :class:`io.BytesIO` to be thread safe.
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