Uses libbz2 c-library using ctypes to decompress files.

dbz2.py

#!/usr/bin/env python3
#
# "THE BEER-WARE LICENSE" (Revision 42):
# <fser@code-libre.org> wrote this file. As long as you retain this notice you
# can do whatever you want with this stuff. If we meet some day, and you think
# this stuff is worth it, you can buy me a beer in return.

from ctypes import *
from ctypes.util import find_library
from sys import argv, stderr, exit

if len(argv) < 2:
    print("Usage: {} <filename>".format(argv[0]), file=stderr)
    exit(1)

z2 = find_library("bz2")
if not z2:
    print("Lib bz2 not found :(")
    exit(1)


libz2 = CDLL(z2)

for infile in argv[1:]:
    outfile = infile[:-4] if infile[-4:] == '.bz2' else infile + '.out'

    f = libz2.BZ2_bzopen(infile.encode(), "r".encode())

    buffer = create_string_buffer(1024)
    ptr = cast(f, c_void_p)

    with open(outfile, 'w') as out:
        while True:
            res = libz2.BZ2_bzread(ptr, buffer, sizeof(buffer))
            if res <= 0:
                break
            out.write(buffer.value[:res].decode())

    libz2.BZ2_bzclose(f)