Python Number to Pretty String (comma adding)

def Num2PrettyString(Num):
    String = str(Num)[::-1]
    Result = ''
    Count = 1
    for i in String:
        if Count == 3:
            Result = Result + i + ','
            Count = 1
        else:
            Result = Result + i
            Count = Count + 1

    Result = Result[::-1]

    if Result.startswith(','):
        Result = Result[1:]

    return Result