[issue22908] ZipExtFile in zipfile can be seekable

[Gregory P. Smith]

Change by Gregory P. Smith :


--
resolution:  -> fixed
stage: patch review -> commit review
status: open -> closed
versions: +Python 3.7 -Python 3.5

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[issue22908] ZipExtFile in zipfile can be seekable

[Gregory P. Smith]

Gregory P. Smith  added the comment:


New changeset 066df4fd454d6ff9be66e80b2a65995b10af174f by Gregory P. Smith 
(John Jolly) in branch 'master':
bpo-22908: Add seek and tell functionality to ZipExtFile (GH-4966)
https://github.com/python/cpython/commit/066df4fd454d6ff9be66e80b2a65995b10af174f


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[issue22908] ZipExtFile in zipfile can be seekable

[Gregory P. Smith]

Change by Gregory P. Smith :


--
assignee: serhiy.storchaka -> gregory.p.smith
nosy: +gregory.p.smith

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[issue22908] ZipExtFile in zipfile can be seekable

[John Jolly]

John Jolly  added the comment:

Please be gentle, this is my first submission to python.

The use case for me was a recursive zip-within-a-zip situation. I wanted to 
allow the creation of a zipfile.ZipFile from an existing zipfile.ZipExtFile, 
but the lack of seek prevented this.

I simply treated forward seeks as a read, and backward seeks as a 
reset-and-read. The reset was the tricky part as it required restoring several 
original values such as the remaining compressed length, remaining data length, 
and the running crc32.

I pushed this into the latest upstream branch, but as I am testing this in v3.4 
it should be easy to backport if necessary (I suspect not).

I based my fix on a little program that I wrote to test the feasibility of this 
idea. I am attaching that test program here.

--
nosy: +jjolly
Added file: https://bugs.python.org/file47345/ziz.py

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[issue22908] ZipExtFile in zipfile can be seekable

[Roundup Robot]

Change by Roundup Robot :


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pull_requests: +4858
stage: needs patch -> patch review

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[issue22908] ZipExtFile in zipfile can be seekable

[Jürgen A . Erhard]

Jürgen A. Erhard added the comment:

To add to this (without looking at the patch): I just to my astonishment 
learned that a ZipExtFile doesn't even support tell().  I can understand the 
seek being nontrivial... but the tell?  It's a bytestream, and there is (isn't 
there?) a clear definition of what next byte a read(1) would deliver.  It 
should be trivial to keep track of the (only ever increasing) file position.

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nosy: +jae

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[issue22908] ZipExtFile in zipfile can be seekable

[Daniel Kessel]

Daniel Kessel added the comment:

It would be great to have the ZipFileExt class seekable.
This would help in implementing features in other projects.

For example, pydicom would gain the ability to read from ZIP files, as 
mentioned in https://github.com/darcymason/pydicom/issues/219

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nosy: +dkessel

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[issue22908] ZipExtFile in zipfile can be seekable

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Actually TarFile already works with non-seekable streams. Use TarFile.open() 
with mode='r|*' or like.

On other hand I'm not against the make non-compressed ZipExtFile seekable. It 
can be helpful in case when ZIP file is used just as a container for other 
files.

--
assignee:  - serhiy.storchaka
stage:  - needs patch
versions: +Python 3.5 -Python 3.4

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[issue22908] ZipExtFile in zipfile can be seekable

[Iridium Yang]

New submission from Iridium Yang:

The ZipExtFile class in zipfile module does not provide a seek method like 
GzipFile. As a result, it is hard to manipulate files without extract all the 
content.
For example, a very large tar file compressed with zip. The TarFile module can 
operate on file object, but need seek method. So the ZipExtFile instance return 
from ZipFile can not passed into TarFile.
This may seem strange but I encounter this on Samsung firmware.

In fact, the seek method in GzipFile or someother compressed format can be 
implemented in zipfile very easily. Here is my naive modification (nearly same 
as in GzipFile)

--
components: Library (Lib)
files: zipfile.diff
keywords: patch
messages: 231438
nosy: Iridium.Yang
priority: normal
severity: normal
status: open
title: ZipExtFile in zipfile can be seekable
type: enhancement
versions: Python 3.4
Added file: http://bugs.python.org/file37237/zipfile.diff

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[issue22908] ZipExtFile in zipfile can be seekable

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I'm -1 on adding the seek method with linear complexity. This looks as 
attractive nonsense to me. It would be better just make TarFile working with 
non-seekable streams.

--
nosy: +serhiy.storchaka

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