Python 语法之操作符和表达式

转载信息：http://blog.chinaunix.net/uid-20393955-id-345384.html

File information

2009-10-23

磁针石：xurongzhong#gmail.com

博客：oychw.cublog.cn

参考资料：

《Python Essential Reference 4th Edition 2009》

《beginning python from novice to professional second edition 2008》

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＊数值运算符

x + y 加法

x - y 减法

x * y 乘法

x / y 除法

x // y 整除

x ** y 乘方

x % y 取模

–x Unary minus

+x Unary plus

Python 2，两个整数相除结果为取模，Python 3，7/4得1，Python 3中改为用浮点运算，得1.75。Python 2导入：from __future__ import division，强制转换。Future一般用于调用以后版本会实现的功能。

>>>2.75 % 0.5

0.25

>>>-3 ** 2

-9

>>> (-3) ** 2

9

长整数的处理：

>>>1000000000000000000

1000000000000000000L

2.2及以前版本处理的范围：2147483647 (or smaller than –2147483648）

16进制：

>>> 0xAF

175

八进制：

>>>010

8

变量必须赋值才能使用。

位操作符：

x << y Left shift

x >> y Right shift

x & y Bitwise and

x | y Bitwise or

x ^ y Bitwise xor (exclusive or)

~x Bitwise negation

python在位运算不会自动截断，要注意到是否会产生巨长的结果。

其他运算符：

abs(x) Absolute value

divmod(x,y) Returns (x // y, x % y)

pow(x,y [,modulo]) Returns (x ** y) % modulo

round(x,[n]) Rounds to the nearest multiple of 10-n (floating-point numbers

only)

注意pow可以作为三元运算符，一般用于加密算法。

round以远离0为目标：round(0.5)得1，round(-0.5)得-1。Python 3中有点怪异：round(0.5)和round(0.5)得0，round(1.5)得2， round(-1.5)得-2。

算数比较符：

x < y Less than

x > y Greater than

x == y Equal to

x != y Not equal to

x >= y Greater than or equal to

x <= y Less than or equal to

它们的连接，比如w < x < y < z，理解为w < x and x < y and y < z

python中的隐式转换并不多。

＊复合运算符

x +=y

x -=y

x *=y

x /=y

x //=y

x **=y

x %=y

x& =y

x |=y

x ^=y

x>> =y

x<< =y

a = 3

b = [1,2]

c = "Hello %s %s"

a += 1 # a = 4

b[1] += 10 # b = [1, 12]

c %= ("Monty", "Python") # c = "Hello Monty Python"

＊点号

＊函数调用（）

def foo(x,y,z):

return x+y+z

from functools import partial

f = partial(foo,1,2)

f(3)

这个东东和currying进程很类似。

＊函数调用（）

def foo(x,y,z):

return x+y+z

from functools import partial

f = partial(foo,1,2)

f(3)

这个东东和currying进程很类似。

＊类型转换

int(x [,base]) Converts x to an integer. base specifies the base if x

is a string.

float(x) Converts x to a floating-point number.

complex(real [,imag]) Creates a complex number.

str(x) Converts object x to a string representation.

repr(x) Converts object x to an expression string.

format(x [,format_spec]) Converts object x to a formatted string.

eval(str) Evaluates a string and returns an object.

tuple(s) Converts s to a tuple.

list(s) Converts s to a list.

set(s) Converts s to a set.

dict(d) Creates a dictionary. d must be a sequence of

(key,value) tuples.

frozenset(s) Converts s to a frozen set.

chr(x) Converts an integer to a character.

unichr(x) Converts an integer to a Unicode character (Python 2

only).

ord(x) Converts a single character to its integer value.

hex(x) Converts an integer to a hexadecimal string.

bin(x) Converts an integer to a binary string.

oct(x) Converts an integer to an octal string.

a = int("34") # a = 34

b = long("0xfe76214", 16) # b = 266822164L (0xfe76214L)

b = float("3.1415926") # b = 3.1415926

c = eval("3, 5, 6") # c = (3,5,6)

＊布尔类型操作符

x or y If x is false, return y; otherwise, return x.

x and y If x is false, return x; otherwise, return y.

not x If x is false, return 1; otherwise, return 0.

＊对象相等

相等：(x == y)

是同一对象：is

＊运算顺序

除了乘方以外，都是由左至右的。

优先级由高到低：

(...), [...], {...} Tuple, list, and dictionary creation

s[i], s[i:j] Indexing and slicing

s.attr Attributes

f(...) Function calls

+x, -x, ~x Unary operators

x ** y Power (right associative)

x * y, x / y, x // y, x % y Multiplication, division, floor division, modulo

x + y, x- y Addition, subtraction

x<< y, x >> y Bit-shifting

x & y Bitwise and

x ^ y Bitwise exclusive or

x | y Bitwise or

x < y, x <= y, Comparison, identity, and sequence membership

tests

x > y, x >= y,

x == y, x != y

x is y, x is not y

x in s, x not in s

not x Logical negation

x and y Logical and

x or y Logical or

lambda args: expr Anonymous function

＊条件表达式

if a< = b:

minvalue = a

else:

minvalue = b

等同于：

minvalue = a if a <=b else b

values = [1, 100, 45, 23, 73, 37, 69 ]

clamped = [x if x < 50 else 50 for x in values]

print(clamped) # [1, 50, 45, 23, 50, 37, 50]