python 之 运算符

Python 运算符

 

Python 运算符

什么是运算符？

本章节主要说明Python的运算符。举个简单的例子 4 +5 = 9 。 例子中，4和5被称为操作数，"+"号为运算符。

Python语言支持以下类型的运算符:

• 算术运算符
• 比较（关系）运算符
• 赋值运算符
• 逻辑运算符
• 位运算符
• 成员运算符
• 身份运算符
• 运算符优先级

接下来让我们一个个来学习Python的运算符。

Python算术运算符

以下假设变量a为10，变量b为20：

运算符	描述	实例
+	加 - 两个对象相加	a + b 输出结果 30
-	减 - 得到负数或是一个数减去另一个数	a - b 输出结果 -10
*	乘 - 两个数相乘或是返回一个被重复若干次的字符串	a * b 输出结果 200
/	除 - x除以y	b / a 输出结果 2
%	取模 - 返回除法的余数	b % a 输出结果 0
**	幂 - 返回x的y次幂	a**b 为10的20次方， 输出结果 100000000000000000000
//	取整除 - 返回商的整数部分	9//2 输出结果 4 , 9.0//2.0 输出结果 4.0

以下实例演示了Python所有算术运算符的操作：

#!/usr/bin/python

a = 21

b = 10

c = 0

c = a + b

print "Line 1 - Value of c is ", c

c = a - b

print "Line 2 - Value of c is ", c

c = a * b

print "Line 3 - Value of c is ", c

c = a / b

print "Line 4 - Value of c is ", c

c = a % b

print "Line 5 - Value of c is ", c

a = 2

b = 3

c = a**b

print "Line 6 - Value of c is ", c

a = 10

b = 5

c = a//b

print "Line 7 - Value of c is ", c

尝试一下 »

以上实例输出结果：

Line 1 - Value of c is 31

Line 2 - Value of c is 11

Line 3 - Value of c is 210

Line 4 - Value of c is 2

Line 5 - Value of c is 1

Line 6 - Value of c is 8

Line 7 - Value of c is 2

Python比较运算符

以下假设变量a为10，变量b为20：

运算符	描述	实例
==	等于 - 比较对象是否相等	(a == b) 返回 False。
!=	不等于 - 比较两个对象是否不相等	(a != b) 返回 true.
<>	不等于 - 比较两个对象是否不相等	(a <> b) 返回 true。这个运算符类似 != 。
>	大于 - 返回x是否大于y	(a > b) 返回 False。
<	小于 - 返回x是否小于y。所有比较运算符返回1表示真，返回0表示假。这分别与特殊的变量True和False等价。注意，这些变量名的大写。	(a < b) 返回 true。
>=	大于等于 - 返回x是否大于等于y。	(a >= b) 返回 False。
<=	小于等于 - 返回x是否小于等于y。	(a <= b) 返回 true。

以下实例演示了Python所有比较运算符的操作：

#!/usr/bin/python

a = 21

b = 10

c = 0

if ( a == b ):

print "Line 1 - a is equal to b"

else:

print "Line 1 - a is not equal to b"

if ( a != b ):

print "Line 2 - a is not equal to b"

else:

print "Line 2 - a is equal to b"

if ( a <> b ):

print "Line 3 - a is not equal to b"

else:

print "Line 3 - a is equal to b"

if ( a < b ):

print "Line 4 - a is less than b"

else:

print "Line 4 - a is not less than b"

if ( a > b ):

print "Line 5 - a is greater than b"

else:

print "Line 5 - a is not greater than b"

a = 5;

b = 20;

if ( a <= b ):

print "Line 6 - a is either less than or equal to  b"

else:

print "Line 6 - a is neither less than nor equal to  b"

if ( b >= a ):

print "Line 7 - b is either greater than  or equal to b"

else:

print "Line 7 - b is neither greater than  nor equal to b"

以上实例输出结果：

Line 1 - a is not equal to b

Line 2 - a is not equal to b

Line 3 - a is not equal to b

Line 4 - a is not less than b

Line 5 - a is greater than b

Line 6 - a is either less than or equal to b

Line 7 - b is either greater than or equal to b

Python赋值运算符

以下假设变量a为10，变量b为20：

运算符	描述	实例
=	简单的赋值运算符	c = a + b 将 a + b 的运算结果赋值为 c
+=	加法赋值运算符	c += a 等效于 c = c + a
-=	减法赋值运算符	c -= a 等效于 c = c - a
*=	乘法赋值运算符	c *= a 等效于 c = c * a
/=	除法赋值运算符	c /= a 等效于 c = c / a
%=	取模赋值运算符	c %= a 等效于 c = c % a
**=	幂赋值运算符	c **= a 等效于 c = c ** a
//=	取整除赋值运算符	c //= a 等效于 c = c // a

以下实例演示了Python所有赋值运算符的操作：

#!/usr/bin/python

a = 21

b = 10

c = 0

c = a + b

print "Line 1 - Value of c is ", c

c += a

print "Line 2 - Value of c is ", c

c *= a

print "Line 3 - Value of c is ", c

c /= a

print "Line 4 - Value of c is ", c

c  = 2

c %= a

print "Line 5 - Value of c is ", c

c **= a

print "Line 6 - Value of c is ", c

c //= a

print "Line 7 - Value of c is ", c

以上实例输出结果：

Line 1 - Value of c is 31

Line 2 - Value of c is 52

Line 3 - Value of c is 1092

Line 4 - Value of c is 52

Line 5 - Value of c is 2

Line 6 - Value of c is 2097152

Line 7 - Value of c is 99864

Python位运算符

按位运算符是把数字看作二进制来进行计算的。Python中的按位运算法则如下：

运算符	描述	实例
&	按位与运算符	(a & b) 输出结果 12 ，二进制解释： 0000 1100
|	按位或运算符	(a | b) 输出结果 61 ，二进制解释： 0011 1101
^	按位异或运算符	(a ^ b) 输出结果 49 ，二进制解释： 0011 0001
~	按位取反运算符	(~a ) 输出结果 -61 ，二进制解释： 1100 0011， 在一个有符号二进制数的补码形式。
<<	左移动运算符	a << 2 输出结果 240 ，二进制解释： 1111 0000
>>	右移动运算符	a >> 2 输出结果 15 ，二进制解释： 0000 1111

以下实例演示了Python所有位运算符的操作：

#!/usr/bin/python

a = 60            # 60 = 0011 1100

b = 13            # 13 = 0000 1101

c = 0

c = a & b;        # 12 = 0000 1100

print "Line 1 - Value of c is ", c

c = a | b;        # 61 = 0011 1101

print "Line 2 - Value of c is ", c

c = a ^ b;        # 49 = 0011 0001

print "Line 3 - Value of c is ", c

c = ~a;           # -61 = 1100 0011

print "Line 4 - Value of c is ", c

c = a << 2;       # 240 = 1111 0000

print "Line 5 - Value of c is ", c

c = a >> 2;       # 15 = 0000 1111

print "Line 6 - Value of c is ", c

以上实例输出结果：

Line 1 - Value of c is 12

Line 2 - Value of c is 61

Line 3 - Value of c is 49

Line 4 - Value of c is -61

Line 5 - Value of c is 240

Line 6 - Value of c is 15

Python逻辑运算符

Python语言支持逻辑运算符，以下假设变量a为10，变量b为20：

运算符	描述	实例
and	布尔"与" - 如果x为False，x and y返回False，否则它返回y的计算值。	(a and b) 返回 true。
or	布尔"或" - 如果x是True，它返回True，否则它返回y的计算值。	(a or b) 返回 true。
not	布尔"非" - 如果x为True，返回False。如果x为False，它返回True。	not(a and b) 返回 false。

以下实例演示了Python所有逻辑运算符的操作：

#!/usr/bin/python

a = 10

b = 20

c = 0

if ( a and b ):

print "Line 1 - a and b are true"

else:

print "Line 1 - Either a is not true or b is not true"

if ( a or b ):

print "Line 2 - Either a is true or b is true or both are true"

else:

print "Line 2 - Neither a is true nor b is true"

a = 0

if ( a and b ):

print "Line 3 - a and b are true"

else:

print "Line 3 - Either a is not true or b is not true"

if ( a or b ):

print "Line 4 - Either a is true or b is true or both are true"

else:

print "Line 4 - Neither a is true nor b is true"

if not( a and b ):

print "Line 5 - Either a is not true or b is  not true or both are not true"

else:

print "Line 5 - a and b are true"

以上实例输出结果：

Line 1 - a and b are true

Line 2 - Either a is true or b is true or both are true

Line 3 - Either a is not true or b is not true

Line 4 - Either a is true or b is true or both are true

Line 5 - Either a is not true or b is  not true or both are not true

Python成员运算符

除了以上的一些运算符之外，Python还支持成员运算符，测试实例中包含了一系列的成员，包括字符串，列表或元组。

运算符	描述	实例
in	如果在指定的序列中找到值返回True，否则返回False。	x 在 y序列中 , 如果x在y序列中返回True。
not in	如果在指定的序列中没有找到值返回True，否则返回False。	x 不在 y序列中 , 如果x不在y序列中返回True。

以下实例演示了Python所有成员运算符的操作：

#!/usr/bin/python

a = 10

b = 20

list = [1, 2, 3, 4, 5 ];

if ( a in list ):

print "Line 1 - a is available in the given list"

else:

print "Line 1 - a is not available in the given list"

if ( b not in list ):

print "Line 2 - b is not available in the given list"

else:

print "Line 2 - b is available in the given list"

a = 2

if ( a in list ):

print "Line 3 - a is available in the given list"

else:

print "Line 3 - a is not available in the given list"

以上实例输出结果：

Line 1 - a is not available in the given list

Line 2 - b is not available in the given list

Line 3 - a is available in the given list

Python身份运算符

身份运算符用于比较两个对象的存储单元

运算符	描述	实例
is	is是判断两个标识符是不是引用自一个对象	x is y, 如果 id(x) 等于 id(y) , is 返回结果 1
is not	is not是判断两个标识符是不是引用自不同对象	x is not y, 如果 id(x) 不等于 id(y). is not 返回结果 1

以下实例演示了Python所有身份运算符的操作：

#!/usr/bin/python

a = 20

b = 20

if ( a is b ):

print "Line 1 - a and b have same identity"

else:

print "Line 1 - a and b do not have same identity"

if ( id(a) == id(b) ):

print "Line 2 - a and b have same identity"

else:

print "Line 2 - a and b do not have same identity"

b = 30

if ( a is b ):

print "Line 3 - a and b have same identity"

else:

print "Line 3 - a and b do not have same identity"

if ( a is not b ):

print "Line 4 - a and b do not have same identity"

else:

print "Line 4 - a and b have same identity"

以上实例输出结果：

Line 1 - a and b have same identity

Line 2 - a and b have same identity

Line 3 - a and b do not have same identity

Line 4 - a and b do not have same identity

Python运算符优先级

以下表格列出了从最高到最低优先级的所有运算符：

运算符	描述
**	指数 (最高优先级)
~ + -	按位翻转, 一元加号和减号 (最后两个的方法名为 +@ 和 -@)
* / % //	乘，除，取模和取整除
+ -	加法减法
>> <<	右移，左移运算符
&	位 'AND'
^ |	位运算符
<= < > >=	比较运算符
<> == !=	等于运算符
= %= /= //= -= += *= **=	赋值运算符
is is not	身份运算符
in not in	成员运算符
not or and	逻辑运算符

以下实例演示了Python所有运算符优先级的操作：

#!/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

以上实例输出结果：

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

知识点总结：

python中会为每个出现的对象分配内存，哪怕他们的值完全相等（注意是相等不是相同）。如执行a=2.0，b=2.0这两个语句时会先后为2.0这个Float类型对象分配内存，然后将a与b分别指向这两个对象。所以a与b指向的不是同一对象：

a=2.0
b=2.0
a is b # 结果为False
a == b # 结果为True

但是为了提高内存利用效率对于一些简单的对象，如一些数值较小的int对象，python采取重用对象内存的办法，如指向a=2，b=2时，由于2作为简单的int类型且数值小，python不会两次为其分配内存，而是只分配一次，然后将a与b同时指向已分配的对象：

a=
b=
a is b # 结果为True
a == b # 结果为True

如但果赋值的不是2而是大的数值，情况就跟前面的一样了：

a=
b=
a is b # 结果为False
a == b # 结果为True

这里is 和==类似编译原理中传值于传地址。又或者说是is只是传递的指针，判断是否指向同一个地址块，这样is两边的参数指向内存中同个地址块，毕竟我家电视跟你电视不是同一个东西。而==则是仅仅判断值相同

如果变量写在同一行,将会指向同一个对象

>>> a=; b=;   # 写在同一行
>>> a is b
True
>>> a == b
True

>>> c =    # 写在不同一行
>>> d =    # 写在不同一行
>>> c is d
False
>>> c == d
True
>>> 

纠正：在2017版本中  不管数字的大小，只要是值相等， a is b=true

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