if 判断条件：
  执行语句……
else：
  执行语句……

#!/usr/bin/python
# -*- coding: UTF-8 -*-

# 例1：if 基本用法

flag = False
name = 'luren'
if name == 'python':     # 判断变量否为'python'
  flag = True     # 条件成立时设置标志为真
  print 'welcome boss'  # 并输出欢迎信息
else:
  print name       # 条件不成立时输出变量名称

>>> luren  # 输出结果

if 判断条件1:
  执行语句1……
elif 判断条件2:
  执行语句2……
elif 判断条件3:
  执行语句3……
else:
  执行语句4……

#!/usr/bin/python
# -*- coding: UTF-8 -*-
# 例2：elif用法

num = 5   
if num == 3:      # 判断num的值
  print 'boss'    
elif num == 2:
  print 'user'
elif num == 1:
  print 'worker'
elif num < 0:      # 值小于零时输出
  print 'error'
else:
  print 'roadman'   # 条件均不成立时输出

>>> roadman # 输出结果

#!/usr/bin/python
# -*- coding: UTF-8 -*-

# 例3：if语句多个条件

num = 9
if num >= 0 and num <= 10:  # 判断值是否在0~10之间
  print 'hello'
>>> hello # 输出结果

num = 10
if num < 0 or num > 10:  # 判断值是否在小于0或大于10
  print 'hello'
else:
 print 'undefine'
>>> undefine # 输出结果

num = 8
# 判断值是否在0~5或者10~15之间
if (num >= 0 and num <= 5) or (num >= 10 and num <= 15):  
  print 'hello'
else:
  print 'undefine'
>>> undefine # 输出结果

#!/usr/bin/python 
# -*- coding: UTF-8 -*-
 
var = 100 
 
if ( var == 100 ) : print "变量 var 的值为100" 
 
print "Good bye!" 

变量 var 的值为100
Good bye!

#!/usr/bin/python

a = 20
b = 10
c = 15
d = 5
e = 0

e = (a + b) * c / d    #( 30 * 15 ) / 5
print "Value of (a + b) * c / d is ", e

e = ((a + b) * c) / d   # (30 * 15 ) / 5
print "Value of ((a + b) * c) / d is ", e

e = (a + b) * (c / d);  # (30) * (15/5)
print "Value of (a + b) * (c / d) is ", e

e = a + (b * c) / d;   # 20 + (150/5)
print "Value of a + (b * c) / d is ", e

Value of (a + b) * c / d is 90
Value of ((a + b) * c) / d is 90
Value of (a + b) * (c / d) is 90
Value of a + (b * c) / d is 50