if 判斷條件：
  執(zhí)行語(yǔ)句……
else：
  執(zhí)行語(yǔ)句……

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

# 例1：if 基本用法

flag = False
name = 'luren'
if name == 'python':     # 判斷變量否為'python'
  flag = True     # 條件成立時(shí)設(shè)置標(biāo)志為真
  print 'welcome boss'  # 并輸出歡迎信息
else:
  print name       # 條件不成立時(shí)輸出變量名稱

>>> luren  # 輸出結(jié)果

if 判斷條件1:
  執(zhí)行語(yǔ)句1……
elif 判斷條件2:
  執(zhí)行語(yǔ)句2……
elif 判斷條件3:
  執(zhí)行語(yǔ)句3……
else:
  執(zhí)行語(yǔ)句4……

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

num = 5   
if num == 3:      # 判斷num的值
  print 'boss'    
elif num == 2:
  print 'user'
elif num == 1:
  print 'worker'
elif num < 0:      # 值小于零時(shí)輸出
  print 'error'
else:
  print 'roadman'   # 條件均不成立時(shí)輸出

>>> roadman # 輸出結(jié)果

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

# 例3：if語(yǔ)句多個(gè)條件

num = 9
if num >= 0 and num <= 10:  # 判斷值是否在0~10之間
  print 'hello'
>>> hello # 輸出結(jié)果

num = 10
if num < 0 or num > 10:  # 判斷值是否在小于0或大于10
  print 'hello'
else:
 print 'undefine'
>>> undefine # 輸出結(jié)果

num = 8
# 判斷值是否在0~5或者10~15之間
if (num >= 0 and num <= 5) or (num >= 10 and num <= 15):  
  print 'hello'
else:
  print 'undefine'
>>> undefine # 輸出結(jié)果

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

變量 var 的值為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