socket.makefile does not handle line buffering #75245

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1. kchen commented on Jul 27, 2017

   

   kchenmannequin

   on Jul 27, 2017

   MannequinAuthor

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   File objects generated with socket.makefile and that attempt to use line buffering appear to not actually use line buffering, at least for writing. In this example, the string does not appear to be written until the flush call.

   First, set up a socket:
   $ nc -l -U /tmp/foo

   Then:

   Python 3.6.2 (default, Jul 26 2017, 01:41:27) 
   [GCC 4.8.4] on linux
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import socket
   >>> s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
   >>> s.connect("/tmp/foo")
   >>> f = s.makefile("rw", buffering=1)
   >>> f.write("asdf\n")
   5
   >>> f.flush()

   The following patch appears to fix the problem:

   --- socket.py.orig	2017-07-25 21:41:39.974554944 -0400
   +++ socket.py	2017-07-27 17:02:58.223353418 -0400
   @@ -253,7 +253,11 @@
                buffer = io.BufferedWriter(raw, buffering)
            if binary:
                return buffer
   -        text = io.TextIOWrapper(buffer, encoding, errors, newline)
   +        line_buffering = False
   +        if buffering == 1:
   +            line_buffering = True
   +        text = io.TextIOWrapper(buffer, encoding, errors, newline,
   +                                line_buffering)
            text.mode = mode
            return text
2. kchen

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jul 27, 2017
3. vxgmichel commented on Mar 16, 2019

   

   vxgmichelmannequin

   on Mar 16, 2019

   Mannequin

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   I ran into this issue too so I went ahead and created a pull request (#12370).
4. vxgmichel

   added

   3.7 (EOL)end of lifeend of life

   3.8 (EOL)end of lifeend of life

   on Mar 16, 2019
5. ezio-melotti

   transferred this issue fromon Apr 10, 2022
6. serhiy-storchaka

   added

   topic-socketRelated to the socket, socketserver and selectors modulesRelated to the socket, socketserver and selectors modules

   on Jul 3, 2026
7. serhiy-storchaka

   added

   type-featureA feature request or enhancementA feature request or enhancement

   3.16new features, bugs and security fixesnew features, bugs and security fixes

   and removed

   3.7 (EOL)end of lifeend of life

   3.8 (EOL)end of lifeend of life

   on Aug 12, 2026
8. serhiy-storchaka commented on Aug 12, 2026

   

   serhiy-storchaka

   on Aug 12, 2026

   Member

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   This worked in Python 2, where makefile() returned socket._fileobject, which implemented buffering itself: bufsize=1 meant line buffering for writing and default buffering for reading, in any mode, because there was no text and binary distinction at that level. It was lost when makefile() was reimplemented on top of the io module, where line buffering only exists in TextIOWrapper.

   #12370 passes line_buffering to TextIOWrapper, so buffering=1 works again in text mode. In binary mode it now emits a RuntimeWarning and uses the default buffer size, as open() does; previously it silently created a 1-byte buffer.

   LineBufferedFileObjectClassTestCase is a leftover from Python 2: it sets bufsize = 1, but uses binary mode, where line buffering has never worked in Python 3. It is replaced with a test in text mode.
9. serhiy-storchaka

   closed this as completedon Aug 12, 2026
10. serhiy-storchaka

    added a commit that references this issue on Aug 12, 2026

    gh-75245: Support line buffering in socket.makefile() (GH-12370)

    ...

    dbac03b
11. mbeijen

    added a commit that references this issue on Aug 14, 2026

    pythongh-75245: Support line buffering in socket.makefile() (pythonGH…

    ...

    89ef690