Unpack zip `filename` to `extract_dir`

def _unpack_zipfile(filename, extract_dir):
    """Unpack zip `filename` to `extract_dir`
    """
    try:
        import zipfile
    except ImportError:
        raise ReadError('zlib not supported, cannot unpack this archive.')

    if not zipfile.is_zipfile(filename):
        raise ReadError("%s is not a zip file" % filename)

    zip = zipfile.ZipFile(filename)
    try:
        for info in zip.infolist():
            name = info.filename

            # don't extract absolute paths or ones with .. in them
            if name.startswith('/') or '..' in name:
                continue

            target = os.path.join(extract_dir, *name.split('/'))
            if not target:
                continue

            _ensure_directory(target)
            if not name.endswith('/'):
                # file
                data = zip.read(info.filename)
                f = open(target, 'wb')
                try:
                    f.write(data)
                finally:
                    f.close()
                    del data
    finally:
        zip.close()