def common_prefix(string_list):
    """Return the common prefix of a list of strings

    TODO: move it into utils package

    :param list(str) string_list: List of strings
    :rtype: str
    """
    prefix = ''
    for ch in zip(string_list):
        c = ch[0]
        good = True
        for i in ch:
            if i != c:
                good = False
                break
        if good:
            prefix += c
        else:
            break
    return prefix
