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Title:Deprecate codecs.StreamReader and codecs.StreamWriter
Last-Modified:2016-05-03 13:52:22 +0300 (Tue, 03 May 2016)
Author:Victor Stinner <victor.stinner at>
Type:Standards Track


io.TextIOWrapper and codecs.StreamReaderWriter offer the same API [1]. TextIOWrapper has more features and is faster than StreamReaderWriter. Duplicate code means that bugs should be fixed twice and that we may have subtle differences between the two implementations.

The codecs module was introduced in Python 2.0 (see the PEP 100). The io module was introduced in Python 2.6 and 3.0 (see the PEP 3116), and reimplemented in C in Python 2.7 and 3.1.

PEP Deferral

Further exploration of the concepts covered in this PEP has been deferred for lack of a current champion interested in promoting the goals of the PEP and collecting and incorporating feedback, and with sufficient available time to do so effectively.


When the Python I/O model was updated for 3.0, the concept of a "stream-with-known-encoding" was introduced in the form of io.TextIOWrapper. As this class is critical to the performance of text-based I/O in Python 3, this module has an optimised C version which is used by CPython by default. Many corner cases in handling buffering, stateful codecs and universal newlines have been dealt with since the release of Python 3.0.

This new interface overlaps heavily with the legacy codecs.StreamReader, codecs.StreamWriter and codecs.StreamReaderWriter interfaces that were part of the original codec interface design in PEP 100. These interfaces are organised around the principle of an encoding with an associated stream (i.e. the reverse of arrangement in the io module), so the original PEP 100 design required that codec writers provide appropriate StreamReader and StreamWriter implementations in addition to the core codec encode() and decode() methods. This places a heavy burden on codec authors providing these specialised implementations to correctly handle many of the corner cases (see Appendix A) that have now been dealt with by io.TextIOWrapper. While deeper integration between the codec and the stream allows for additional optimisations in theory, these optimisations have in practice either not been carried out and else the associated code duplication means that the corner cases that have been fixed in io.TextIOWrapper are still not handled correctly in the various StreamReader and StreamWriter implementations.

Accordingly, this PEP proposes that:

  • be updated to delegate to the builtin open() in Python 3.3;
  • the legacy codecs.Stream* interfaces, including the streamreader and streamwriter attributes of codecs.CodecInfo be deprecated in Python 3.3.


StreamReader and StreamWriter issues

  • StreamReader is unable to translate newlines.
  • StreamWriter doesn't support "line buffering" (flush if the input text contains a newline).
  • StreamReader classes of the CJK encodings (e.g. GB18030) only supports UNIX newlines ('\n').
  • StreamReader and StreamWriter are stateful codecs but don't expose functions to control their state (getstate() or setstate()). Each codec has to handle corner cases, see Appendix A.
  • StreamReader and StreamWriter are very similar to IncrementalReader and IncrementalEncoder, some code is duplicated for stateful codecs (e.g. UTF-16).
  • Each codec has to reimplement its own StreamReader and StreamWriter class, even if it's trivial (just call the encoder/decoder).
  •, "r") creates an io.TextIOWrapper object.
  • No codec implements an optimized method in StreamReader or StreamWriter based on the specificities of the codec.

Issues in the bug tracker:

  • Issue #5445 (2009-03-08): codecs.StreamWriter.writelines problem when passed generator
  • Issue #7262: (2009-11-04): + eol (windows)
  • Issue #8260 (2010-03-29): When I use and f.readline() follow up by return bad result
  • Issue #8630 (2010-05-05): Keepends param in codec readline(s)
  • Issue #10344 (2010-11-06): codecs.readline doesn't care buffering
  • Issue #11461 (2011-03-10): Reading UTF-16 with codecs.readline() breaks on surrogate pairs
  • Issue #12446 (2011-06-30): StreamReader Readlines behavior odd
  • Issue #12508 (2011-07-06): Codecs Anomaly
  • Issue #12512 (2011-07-07): codecs: StreamWriter issues with stateful codecs after a seek or with append mode
  • Issue #12513 (2011-07-07): codec.StreamReaderWriter: issues with interlaced read-write

TextIOWrapper features

  • TextIOWrapper supports any kind of newline, including translating newlines (to UNIX newlines), to read and write.
  • TextIOWrapper reuses codecs incremental encoders and decoders (no duplication of code).
  • The io module (TextIOWrapper) is faster than the codecs module (StreamReader). It is implemented in C, whereas codecs is implemented in Python.
  • TextIOWrapper has a readahead algorithm which speeds up small reads: read character by character or line by line (io is 10x through 25x faster than codecs on these operations).
  • TextIOWrapper has a write buffer.
  • TextIOWrapper.tell() is optimized.
  • TextIOWrapper supports random access (read+write) using a single class which permit to optimize interlaced read-write (but no such optimization is implemented).

TextIOWrapper issues

  • Issue #12215 (2011-05-30): TextIOWrapper: issues with interlaced read-write

Possible improvements of StreamReader and StreamWriter

By adding codec state read/write functions to the StreamReader and StreamWriter classes, it will become possible to fix issues with stateful codecs in a base class instead of in each stateful StreamReader and StreamWriter classes.

It would be possible to change StreamReader and StreamWriter to make them use IncrementalDecoder and IncrementalEncoder.

A codec can implement variants which are optimized for the specific encoding or intercept certain stream methods to add functionality or improve the encoding/decoding performance. TextIOWrapper cannot implement such optimization, but TextIOWrapper uses incremental encoders and decoders and uses read and write buffers, so the overhead of incomplete inputs is low or nul.

A lot more could be done for other variable length encoding codecs, e.g. UTF-8, since these often have problems near the end of a read due to missing bytes. The UTF-32-BE/LE codecs could simply multiply the character position by 4 to get the byte position.

Usage of StreamReader and StreamWriter

These classes are rarely used directly, but indirectly using They are not used in Python 3 standard library (except in the codecs module).

Some projects implement their own codec with StreamReader and StreamWriter, but don't use these classes.

Backwards Compatibility

Keep the public API, can be replaced by the builtin open() function. open() has a similar API but has also more options. Both functions return file-like objects (same API). was the only way to open a text file in Unicode mode until Python 2.6. Many Python 2 programs uses this function. Removing implies more work to port programs from Python 2 to Python 3, especially projets using the same code base for the two Python versions (without using 2to3 program). is kept for backward compatibility with Python 2.

Deprecate StreamReader and StreamWriter

Instanciating StreamReader or StreamWriter must emit a DeprecationWarning in Python 3.3. Defining a subclass doesn't emit a DeprecationWarning. will be changed to reuse the builtin open() function (TextIOWrapper) to read-write text files.

Alternative Approach

An alternative to the deprecation of the codecs.Stream* classes is to rename to codecs.open_stream(), and to create a new function reusing open() and so io.TextIOWrapper.

Appendix A: Issues with stateful codecs

It is difficult to use correctly a stateful codec with a stream. Some cases are supported by the codecs module, while io has no more known bug related to stateful codecs. The main difference between the codecs and the io module is that bugs have to be fixed in StreamReader and/or StreamWriter classes of each codec for the codecs module, whereas bugs can be fixed only once in io.TextIOWrapper. Here are some examples of issues with stateful codecs.

Stateful codecs

Python supports the following stateful codecs:

  • cp932
  • cp949
  • cp950
  • euc_jis_2004
  • euc_jisx2003
  • euc_jp
  • euc_kr
  • gb18030
  • gbk
  • hz
  • iso2022_jp
  • iso2022_jp_1
  • iso2022_jp_2
  • iso2022_jp_2004
  • iso2022_jp_3
  • iso2022_jp_ext
  • iso2022_kr
  • shift_jis
  • shift_jis_2004
  • shift_jisx0213
  • utf_8_sig
  • utf_16
  • utf_32

Read and seek(0)

with open(filename, 'w', encoding='utf-16') as f:
    assert == 'abcdef'
    assert == 'abcdef'

The io and codecs modules support this usecase correctly.


with open(filename, 'w', encoding='utf-16') as f:
    pos = f.tell()
with open(filename, 'w', encoding='utf-16') as f:
with open(filename, 'r', encoding='utf-16') as f:
    assert == '###def'

The io module supports this usecase, whereas codecs fails because it writes a new BOM on the second write (issue #12512).

Append mode

with open(filename, 'w', encoding='utf-16') as f:
with open(filename, 'a', encoding='utf-16') as f:
with open(filename, 'r', encoding='utf-16') as f:
    assert == 'abcdef'

The io module supports this usecase, whereas codecs fails because it writes a new BOM on the second write (issue #12512).


[1]StreamReaderWriter has two more attributes than TextIOWrapper, reader and writer.