P7 if 条件 P8 if else 条件 P9 if elif else

P7 if条件 

x = 1
y = 2

if x > y:
    print('x is greater than y.')

输出为empty

x = 1
y = 2

if x < y:
    print('x is less than y.')

输出为x is less than y.

<

>

<=

>=

!=

==        注意等于不是一个=而是==

P8  if else 条件

x = 1
y = 2

if x > y:
    print('x is greater than y.')
else:
    print('x is less than or equal to y.')

注意if和else语句都以冒号结尾

注意格式

P9 if elif else

x = 1

if x > 1:
    print('x > 1')
elif x < -1:
    print('x < -1')
elif x < 1:
    print('x < 1')
else:
    print('x = 1')
print('finish running')

当满足其中一个条件后就会跳出if，继续执行后面的程序