class J:
    """
    Sequence Join Format String Wrapper
    
    >>> seq = ['a', 'b', 1, None]
    >>> print(f"default: {J(seq)}"
    default: 1,b,1,None
    >>> print(f"custom-delimeter: {J(seq):/-}"
    custom-delimiter: a/-b/-1/-None
    """
    def __init__(self, seq):
        self.seq = seq
    
    def __format__(self, format_spec=','):
        # format_spec is the delimiter to use
        if self.seq:
            return format_spec.join(str(item) for item in self.seq)
        else:
            return ''