is_zipfile does not set the position to the file header

[0xMattina]

cpython/Lib/zipfile/__init__.py

Lines 243 to 244 in 4e75509

	if hasattr(filename, "read"):
	result = _check_zipfile(fp=filename)

When filename is BytesIO, is_zipfile don't set the position to the file header.
This will affect other codes.

And it's my first issues,i don't known how to report the question,sorry

Linked PRs

• gh-122356: restore the position of a file-like object after zipfile.is_zipfile #122397

[terryjreedy]

Can you add test code that demonstrates the claimed bug? That prints a result that you claim is buggy (different from correct)?

[0xMattina]

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Test code:

import zipfile
from io import BytesIO

if __name__ == '__main__':
    with open(r"d:\test.zip", "rb") as fp_file:
        fp_io = BytesIO(fp_file.read())
    print("before pos:", fp_io.tell())
    print("is_zipfile:", zipfile.is_zipfile(fp_io))
    print("after pos:", fp_io.tell())

Output:

before pos: 0
is_zipfile: True
after pos: 122

[ronaldoussoren]

The documentation for is_zipfile does not promise anything about the file position after the call. That makes this a feature request.

What would be the advantage of ensuring that the file position is at a particular place in the file after calling this function?

[0xMattina]

Test code:

import tarfile
import zipfile
from io import BytesIO

if __name__ == '__main__':
    with open(r"d:\1.zip", "rb") as fp_file:
        fp_io = BytesIO(fp_file.read())

    print("before <is_tarfile> pos:", fp_io.tell())
    print("<is_tarfile> result:", tarfile.is_tarfile(fp_io))
    print("after <is_tarfile> pos:", fp_io.tell())
    print()
    print("before <is_zipfile> pos:", fp_io.tell())
    print("<is_zipfile> result:", zipfile.is_zipfile(fp_io))
    print("after <is_zipfile> pos:", fp_io.tell())

Output:

before <is_tarfile> pos: 0
<is_tarfile> result: False
after <is_tarfile> pos: 0

before <is_zipfile> pos: 0
<is_zipfile> result: True
after <is_zipfile> pos: 122

I tried some other libraries, and tarfile is a std library, it will set pos to the value before call.
I think if zipfile does the same, it will improve the logic and consistency of the code.

[picnixz]

For tarfile, it was fixed in #26488 so I think we should also fix it for zipfile (but I'll mark it as a feature because the tarfile one was not backported to 3.9 or 3.10, so I think it should be the same for this one).

[0xMattina]

Yes，that's what i want.Thanks!

[picnixz]

By the way, we should have a look at #72680 and #5053 as well since this seems related (perhaps we should first consider fixing the false positives before fixing the position).

[Gordonei]

First time commenter, so apologies if I've missed something obvious.

I found this issue while debugging some code that made use of mock_open to test code that used is_zipfile. In that case, it either reset the position of the header or didn't alter it (more likely the latter), so the mocked behaviour diverged from normal filelike behaviours.

Should I create a separate issue for this, or leave it, since it will be fixed by this for filelike files by @picnixz's PR?