Add data_offset field to ZipInfo

[zhangxp1998]

BPO	45006
Nosy	@ammaraskar, @zhangxp1998
PRs	bpo-45006 Add a data_offset field to ZipInfo #27961

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-08-26.02:30:22.624>
labels = ['type-feature', 'library', '3.11']
title = 'Add data_offset field to ZipInfo'
updated_at = <Date 2021-10-06.15:22:47.417>
user = 'https://github.com/zhangxp1998'

bugs.python.org fields:

activity = <Date 2021-10-06.15:22:47.417>
actor = 'ammar2'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-26.02:30:22.624>
creator = 'zhangxp1998'
dependencies = []
files = []
hgrepos = []
issue_num = 45006
keywords = ['patch']
message_count = 2.0
messages = ['400306', '403316']
nosy_count = 2.0
nosy_names = ['ammar2', 'zhangxp1998']
pr_nums = ['27961']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45006'
versions = ['Python 3.11']

[zhangxp1998]

Currently python's zipfile module does not have a way query starting offset of compressed data. This might be handy when the user wants to copy compressed data as is. Therefore I propose adding a data_offset field to zipfile.ZipInfo, which stores the offset to beginning of compressed data.

[ammaraskar]

Could you explain your use-case for this feature in a bit more detail? zipfile is meant to be a relatively high level library to do common tasks such as reading/writing/listing files.

The use case for data_offset proposed here seems to be relatively advanced and I don't see how it would be to useful for the vast majority of users.

(Without adding to the public API, I think you can achieve a pretty similar functionality by using the following)

compressed_data = zipfile.open(zipinfo)._read2(compressed_size)

Obviously, this relies on undocumented internals, but for a niche use case that might not be the worst thing:

cpython/Lib/zipfile.py

Lines 1042 to 1056 in 61892c0

	def _read2(self, n):
	if self._compress_left <= 0:
	return b''
	n = max(n, self.MIN_READ_SIZE)
	n = min(n, self._compress_left)
	data = self._fileobj.read(n)
	self._compress_left -= len(data)
	if not data:
	raise EOFError
	if self._decrypter is not None:
	data = self._decrypter(data)
	return data

[emmatyping]

I believe this is a duplicate of #84481. I do think adding an offset is useful for several reasons, as outlined in the other issue.

[ammaraskar]

Thanks, closing in favor of the other issue with more discussion. The original issue author here never stated their use case.