[Request] New open() method

[pshelly]

Hello and thanks for making these bindings!
An open() method like gzip module in the python stdlib will be extremely useful.

[indygreg]

I like the feature suggestion. A prerequisite is full implementations of io.RawIOBase on our compressor/decompressor stream readers/writers. We have partial coverage of that today. I'd like to get full coverage in the next release.

[ashwinvis]

For *.tar.zst files, this works for me, at least for reading.

from contextlib import contextmanager
from tarfile import TarFile
import zstandard as zstd


@contextmanager
def open_tar_zst(path_tar_zst):
    """Decompress and open a .tar.zst file"""
    with open(path_tar_zst, 'rb') as fh:
        dctx = zstd.ZstdDecompressor()
        with dctx.stream_reader(fh) as stream:
            yield TarFile(fileobj=stream)


with open_tar_zst("my.tar.zst") as tar:
    tar.list()

[ashwinvis]

FYI, the following methods of TarFile works:

• next
• getmember
• getmembers
• extractall

What does not work:

• extract

In [75]: with open_tar_zst(tarball) as tar: 
    ...:     tar.extract('SIZE') 
    ...:                                                                                                                                                                                      
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-75-8b5ebe57f6d8> in <module>
      1 with open_tar_zst(tarball) as tar:
----> 2     tar.extract('SIZE')
      3 

/usr/lib/python3.8/tarfile.py in extract(self, member, path, set_attrs, numeric_owner)
   2063 
   2064         try:
-> 2065             self._extract_member(tarinfo, os.path.join(path, tarinfo.name),
   2066                                  set_attrs=set_attrs,
   2067                                  numeric_owner=numeric_owner)

/usr/lib/python3.8/tarfile.py in _extract_member(self, tarinfo, targetpath, set_attrs, numeric_owner)
   2135 
   2136         if tarinfo.isreg():
-> 2137             self.makefile(tarinfo, targetpath)
   2138         elif tarinfo.isdir():
   2139             self.makedir(tarinfo, targetpath)

/usr/lib/python3.8/tarfile.py in makefile(self, tarinfo, targetpath)
   2174         """
   2175         source = self.fileobj
-> 2176         source.seek(tarinfo.offset_data)
   2177         bufsize = self.copybufsize
   2178         with bltn_open(targetpath, "wb") as target:

ValueError: cannot seek zstd decompression stream backwards

• gettarinfo

In [98]: with open_tar_zst(tarball) as tar: 
    ...:     print(tar.gettarinfo()) 
    ...:                                                                                                                                                                                      
---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-98-43c3d95a8702> in <module>
      1 with open_tar_zst(tarball, 'rb') as tar:
----> 2     print(tar.gettarinfo())
      3 

/usr/lib/python3.8/tarfile.py in gettarinfo(self, name, arcname, fileobj)
   1804            string.
   1805         """
-> 1806         self._check("awx")
   1807 
   1808         # When fileobj is given, replace name by

/usr/lib/python3.8/tarfile.py in _check(self, mode)
   2382             raise OSError("%s is closed" % self.__class__.__name__)
   2383         if mode is not None and self.mode not in mode:
-> 2384             raise OSError("bad operation for mode %r" % self.mode)
   2385 
   2386     def _find_link_target(self, tarinfo):

OSError: bad operation for mode 'r'