if用于条件判断，如果满足条件就执行否则就继续直到结束，熟悉shell条件判断就觉得so easy
格式：
if condition1:
execute command
elif condition2:
execute command
else:
execute command
for n in condition:
execute command
if示例1： #基于windows版 python3.5
#!/usr/bin/env pyhon
# -*- coding: UTF-8 -*-
# authors: jack
age = 28
guess_num = int(input("INPUT YOUR GUESS NUM:"))
if guess_num == age:
print("Your guess right.GOOD!!!")
elif guess_num > age:
print("Your guess letter!")
else:
print("Your guess smaller!")if示例2：
#!/usr/bin/env pyhon
# -*- coding: UTF-8 -*-
# authors: jack
user = 'jack'
passwd = 'jack123'
username = input("username:")
password = input("password:")
if user == username and password == passwd:
print ("username is correct...,login success!!!")
else:
print("Invaild username or password error...")
for示例1：
#!/usr/bin/env pyhon
# -*- coding: UTF-8 -*-
# authors: jack
age = 28
for n in range(10):
if n < 3:
guess_num = int(input("INPUT YOUR GUESS NUM:"))
if guess_num == age:
print("Your guess right.GOOD!!!")
break #跳出整个loop
elif guess_num > age:
print("Your guess letter!")
else:
print("Your guess smaller!")
else:
print("too many attempts ...bye bye")
break
