2019-04-15:判断一个输入的句子中有多少个字母

def statistics_Letters():
    while 1:
        letters_num=0
        input_sentence = input("请输入你的句子,输入q or Q则退出程序:")
        if len(input_sentence )==1:
            if input_sentence=="q" or input_sentence=="Q":
                return None
        for i in input_sentence:
            if "a"<=i<="z" or "A"<=i<="Z":
                letters_num+=1
        print(letters_num)
print(statistics_Letters())