表达式：运算符和操作数构成的序列
#### 条件判断if
~~~
age = 3
if age >= 18:
print('your age is', age)
print('adult')
elif age <=10:
print('your age is', age)
print('teenager')
else:
print('your age is', age)
print('teenager')
~~~
### 循环for，用于遍历
~~~
sum = 0
for x in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
sum = sum + x
else:
print('end')
for x in range(0,10,1):
print(x)
~~~
### 循环while,用于递归
~~~
sum = 0
while sum > 0:
sum = sum + 1
else:
print('end')
~~~
