[New-bugs-announce] [issue44289] tarfile.is_tarfile() modifies file object's current position

[Andrzej Mateja]

New submission from Andrzej Mateja <mateja.and at gmail.com>:

Since Python 3.9 tarfile.is_tarfile accepts not only paths but also files and file-like objects (bpo-29435).

Verification if a file or file-like object is a tar file modifies file object's current position.

Imagine a function listing names of all tar archive members but checking first if this is a valid tar archive. When its argument is a str or pathlib.Path this is quite straightforward. If the argument is a file of file-like object then current position must be reset or TarFile.getmembers() returns empty list.

import tarfile


def list_tar(archive):
    if tarfile.is_tarfile(archive):
        kwargs = {'fileobj' if hasattr(archive, 'read') else 'name': archive}
        t = tarfile.open(**kwargs)
        return [member.name for member in t.getmembers()]
    return []


if __name__ == '__main__':
    path = 'archive.tar.gz'
    print(list_tar(path))
    print(list_tar(open(path, 'rb')))


['spam.py', 'ham.py', 'bacon.py', 'eggs.py']
[]

----------
components: Library (Lib)
messages: 394922
nosy: mateja.and
priority: normal
severity: normal
status: open
title: tarfile.is_tarfile() modifies file object's current position
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44289>
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