def chunk_str(s, n, char):
    """Insert `char` character every `n` characters in string `s`.

    Canonically pronounced "chunkster".

    """
    # Modified from http://stackoverflow.com/a/312464/3776794
    if n < 1:
        raise ValueError(
            "Cannot split string into chunks with n=%d. n must be >= 1." % n)
    return char.join((s[i:i+n] for i in range(0, len(s), n)))