选修课作业，求大神帮忙

Str = input("请输入只包含字母的字符串：")
d={}
for i in Str:
    d[i] = Str.count(i)
print(d)

def count_letter_num(target_str):
    result_dict = {}
    for i in target_str.lower():
        if i not in result_dict:
            result_dict[i] = 1
        else:
            result_dict[i] += 1
    return result_dict


str_target = input("请输入：")
print(count_letter_num(str_target))

t = input("请输入：").lower()

d = {i:t.count(i) for i in t}

print(d)

t=input("请输入：").lower()
count={}
for i in t:
    count.setdefault(i,0)
    count[i]+=1
print(count)