使用一个变量all_students保存一个班的学生信息(4个)，每个学生需要保存：姓名、年龄、成绩、电话

all_students = [
    {'name':'stu1', 'age': 19, 'score':81, 'tel':'192222'},
    {'name':'stu2', 'age': 29, 'score':90, 'tel':'211222'},
    {'name':'stu3', 'age': 12, 'score':67, 'tel':'521114'},
    {'name':'stu4', 'age': 30, 'score':45, 'tel':'900012'},
] 

1.添加学生：输入学生信息，将输入的学生的信息保存到all_students中
例如输入：
姓名: 小明
年龄: 20
成绩: 100
电话: 111922
那么就在all_students中添加{'name':'小明', 'age': 20, 'score': 100, 'tel':'111922'}

dict_students = {}
name = input('姓名:')
age = input('年龄:')
score = input('成绩:')
tel = input('电话:')
dict_students['name'] = name
dict_students['age'] = age
dict_students['score'] = score
dict_students['tel'] = tel
all_students.append(dict_students)
print(all_students)

结果如下：
[{'name': 'stu1', 'age': 19, 'score': 81, 'tel': '192222'}, 
{'name': 'stu2', 'age': 29, 'score': 90, 'tel': '211222'}, 
{'name': 'stu3', 'age': 12, 'score': 67, 'tel': '521114'}, 
{'name': 'stu4', 'age': 30, 'score': 45, 'tel': '900012'}, 
{'name': 'stu5', 'age': '20', 'score': '95', 'tel': '123456'}]

2.按姓名查看学生信息：
例如输入:
姓名: stu1 就打印：'name':'stu1', 'age': 19, 'score':81, 'tel':'192222'

name_in = input('学生姓名')
for index in range(0,len(all_students)):
    if all_students[index]['name'] == name_in:
        print(all_students[index])

3.求所有学生的平均成绩和平均年龄

sum_score = 0; sum_age = 0
for index in range(0, len(all_students)):
    sum_score += all_students[index]['score']
    sum_age += all_students[index]['age']
print('学生的平均成绩是%.2f,平均年龄是%.2f' %((sum_score)/len(all_students), (sum_age)/len(all_students)))

结果如下：
学生的平均成绩是70.75,平均年龄是22.50

。。。。。

sum_score=0
sum_age=0
for index in range(len(all_students)):
    sum_score+=all_students[index]['score']
    sum_age+=all_students[index]['age']
ave_score=sum_score/len(all_students)
ave_age=sum_age/len(all_students)
print("平均成绩是%.2f,平均年龄是%.2f"%(ave_score,ave_age))

4.删除班级中年龄小于18岁的学生

for index in range(0, len(all_students)):
    if all_students[index]['age'] < 18:
        del all_students[index]['name']
        del all_students[index]['age']
        del all_students[index]['score']
        del all_students[index]['tel']
print(all_students)
结果如下:[{'name': 'stu1', 'age': 19, 'score': 81, 'tel': '192222'}, {'name': 'stu2', 'age': 29, 'score': 90, 'tel': '211222'}, {}, {'name': 'stu4', 'age': 30, 'score': 45, 'tel': '900012'}]

5.统计班级中不及格的学生的人数

count = 0
for index in range(0, len(all_students)):
    if all_students[index]['score'] < 60:
        count += 1
print(count)
结果如下:
1

6.打印手机号最后一位是2的学生的姓名

for index in range(len(all_students)):
    student=all_students[index]
    tel= int(student['tel'])
    if tel %10==2:
        print(student['name'])
输出结果为：
stu1
stu2
stu4