Cannot add empty directory structures to output.

[noisysoil]

Describe the bug
When an archive structure in memory contains empty directories such as:
empty-dir1/empty-dir2/empty-dir3
...then when adding these to an archive, they do not appear in the final output.

To Reproduce
Create a .7z file with an empty directory structure; taking the example above ('empty-dir1/empty-dir2/empty-dir3'):
7z a empty-dir-test.7z empty-dir1

Then recompress the test file with py7zr:

            compression_filters = [{'id': py7zr.FILTER_LZMA2, 'preset': 9}]
            with py7zr.SevenZipFile(f"test-output.7z", mode="w",
                                    filters=compression_filters) as target:
                with py7zr.SevenZipFile(f"empty-dir-test.7z", mode='r') as source:
                    target.writed(source.readall())

Expected behavior
Directory structure should be preserved.

Environment (please complete the following information):

• OS: Kubuntu Linux 21.10
• Python 3.9
• py7zr version: 0.17.3

Test data(please attach in the report):
An archive structure containing an empty directory (see 'To reproduce').

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days

[miurahr]

readall method returns Dict[filename: str, object: io.BytesIO] that cannot return a directory as entry. Are there any idea to express directory?

[Ru13en]

Hi,
There is a way to write empty directories and maintain their structure:

import py7zr
import tempfile
with py7zr.SevenZipFile(f"test-output.7z", mode="w") as target:
    target.write(tempfile.mkdtemp(), "empty-dir1/empty-dir2/empty-dir3")

It should be explained in the docs.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days