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']
[]

This affects more use cases than just is_tarfile() and getmembers() results.

is_tarfile() calls open() which is the root cause of the issue. Calling open() 2+ times will also cause the same issue.

In addition to getmembers(), extracting the tar will also silently fail. (and possibly other operations).

I've suggested a different fix in the comment on the PR:
https://github.com/python/cpython/pull/26488#issuecomment-989367707

New changeset 128ab092cad984b73a117f58fa0e9b4105051a04 by Andrzej Mateja in branch 'main':
bpo-44289: Keep argument file object's current position in tarfile.is_tarfile (GH-26488)
https://github.com/python/cpython/commit/128ab092cad984b73a117f58fa0e9b4105051a04