for循环：用户按照顺序循环可迭代对象的内容
1. for循环字符串
```Python
msg = "string"
for i in msg:
print(i)
```
执行结果为：
```Python
s
t
r
i
n
g
```
2. for循环列表
```Python
lst = ["a", "b", "c"]
for i in lst:
print(i)
```
执行结果为：
```Python
a
b
c
```
3. for循环字典
```Python
dic = {
"name": "yang",
"age": 18,
"sex": "male"
}
for key, value in dic.items():
print(key, value)
```
执行结果为：
```Python
name yang
age 18
sex male
```
