def istextfile(fname, blocksize=512):
    """ Uses heuristics to guess whether the given file is text or binary,
        by reading a single block of bytes from the file.
        If more than 30% of the chars in the block are non-text, or there
        are NUL ('\x00') bytes in the block, assume this is a binary file.
    """
    with open(fname, "rb") as fobj:
        block = fobj.read(blocksize)

    if not block:
        # An empty file is considered a valid text file
        return True

    if b"\x00" in block:
        # Files with null bytes are binary
        return False

    # Use translate's 'deletechars' argument to efficiently remove all
    # occurrences of TEXT_CHARS from the block
    nontext = block.translate(None, TEXT_CHARS)
    return float(len(nontext)) / len(block) <= 0.30