Trying to create an archive with the tarfile module, by specifying a different blocking factor, doesn't seem to work as only the default value is being used. The issue is reproducible on all the active python branches.

Attaching a script to reproduce it.

Original bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1492157

Seems like bufsize is used only in streaming modes. Even in the documentation bufsize is described only in the context of streaming modes. 

Even TarFile constructor doesn't take bufsize as an argument. Why is it so?

Sorry. My bad. There *is* an argument 'copybufsize' in TarFile.

Extract of tartest.py:

tarfile.open(name=_tmp_tar, mode='w', bufsize=EXPECTED_SIZE)

Hum, the bufsize argument only applies to the "...|compression" modes (ex: "w|gz"). Moreover, it shouldn't change the size of the tarfile, it only changes the size of parameters of read() and write() arguments. 

Extract of Stream.__write():

    while len(self.buf) > self.bufsize:
        self.fileobj.write(self.buf[:self.bufsize])
        self.buf = self.buf[self.bufsize:]

> Trying to create an archive with the tarfile module, by specifying a different blocking factor, (...)

I didn't know that we can change the "blocking factor". In the UNIX tar command, I see the -b option:

       -b, --blocking-factor=BLOCKS
              Set record size to BLOCKSx512 bytes.


In Lib/tarfile.py, I see:

BLOCKSIZE = 512                 # length of processing blocks
RECORDSIZE = BLOCKSIZE * 20     # length of records

So values are hardcoded in Python.


So this issue is a feature request, not a bug report :-)