[issue13881] Stream encoder for zlib_codec doesn't use the incremental encoder

[Andrew McNabb]

New submission from Andrew McNabb <amcnabb at mcnabbs.org>:

The stream encoder for the zlib_codec doesn't use the incremental encoder, so it has limited usefulness in practice. This is easiest to show with an example.

Here is the behavior with the stream encoder:

>>> filelike = io.BytesIO()
>>> wrapped = codecs.getwriter('zlib_codec')(filelike)
>>> wrapped.write(b'hello')
>>> filelike.getvalue()
b'x\x9c\xab\x00\x00\x00y\x00y'
>>> wrapped.write(b'x')
>>> filelike.getvalue()
b'x\x9c\xab\x00\x00\x00y\x00yx\x9c\xab\x00\x00\x00y\x00y'
>>>

However, this is the behavior of the incremental encoder:

>>> ienc = codecs.getincrementalencoder('zlib_codec')()
>>> ienc.encode(b'x')
b'x\x9c'
>>> ienc.encode(b'x', final=True)
b'\xab\xa8\x00\x00\x01j\x00\xf1'
>>>

The stream encoder is apparently encoding each write as an individual block, but the incremental encoder buffers until it gets a block that's large enough to be meaningfully compressed.

Fixing this may require addressing a separate issue with stream encoders. Unlike with the GzipFile module, closing a stream encoder closes the underlying file. If this underlying file is a BytesIO, then closing makes it free its buffer, making it impossible to get at the completed file.

----------
components: IO
messages: 152029
nosy: amcnabb
priority: normal
severity: normal
status: open
title: Stream encoder for zlib_codec doesn't use the incremental encoder
type: behavior
versions: Python 3.2

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13881>
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