python学习笔记3---变量与运算符

摘要：什么是变量假设两个做数学运算先把乘以，然后加上，最后再加上列表变量名的命名规则变量名命名只能使用字母数字下划线变量名的首字母不能是数字系统关键字，不能用在变量名中保留关键字不是系统保留关键字，但是不建议作为变量名，否则极易出错动态语言的特性

假设两个list做数学运算

>>> [1,2,3,4,5,6] [1,2,3]
Traceback (most recent call last):
  File "", line 1, in 
    [1,2,3,4,5,6] [1,2,3]
TypeError: list indices must be integers or slices, not tuple

//A B,先把A乘以3，然后加上B，最后再加上列表A
>>> [1,2,3,4,5,6]*3+[1,2,3]+[1,2,3,4,5,6]
[1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6, 1, 2, 3, 1, 2, 3, 4, 5, 6]

>>> A = [1,2,3,4,5,6]
>>> print(A)
[1, 2, 3, 4, 5, 6]
>>> B = [1,2,3]
>>> A*3 + B + A
[1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6, 1, 2, 3, 1, 2, 3, 4, 5, 6]

变量名命名只能使用字母、数字、下划线

>>> 1a = 2   //变量名的首字母不能是数字
SyntaxError: invalid syntax
>>> A2 = "1"
>>> _2 = "1"
>>> A*B="1"
SyntaxError: can"t assign to operator  

系统关键字，不能用在变量名中 保留关键字

>>> and = 1
SyntaxError: invalid syntax
>>> if = 2
SyntaxError: invalid syntax
>>> import = 3
SyntaxError: invalid syntax
>>> type = 3   //type不是系统保留关键字，但是不建议作为变量名，否则极易出错
>>> print(type)
3
>>> type = 1
>>> type(1)
Traceback (most recent call last):
  File "", line 1, in 
    type(1)
TypeError: "int" object is not callable
>>> 1(1)
Traceback (most recent call last):
  File "", line 1, in 
    1(1)
TypeError: "int" object is not callable

python动态语言的特性，声明时不需要指明变量类型

>>> a = "1"
>>> a = 1
>>> a = (1,2,3)
>>> a = {1,2,3}

int、str、tuple是值类型（不可变），list、set、dict是引用类型（可变）

1.int

>>> a = 1
>>> b = a
>>> a = 3
>>> print(b)
1

2.list

>>> a = [1,2,3,4,5]
>>> b = a
>>> a[0] = "1"
>>> print(a)
["1", 2, 3, 4, 5]
>>> print(b)
["1", 2, 3, 4, 5]
>>> a = [1,2,3]
>>> id(a)
4405825224
>>> hex(id(a))
"0x1069b8ec8"
>>> a[0]="1"
>>> id(a)
4405825224
>>> 

3.str

>>> a = "hello"
>>> a = a + "python"  //a加上一个新的字符串，不再是原来的字符串了
>>> print(a)
hellopython
>>> b = "hello"
>>> id(b)
4405534032
>>> b = b + "python"  //加上新的字符串后，id改变
>>> id(b)
4355329456
>>> "python"[0]
"p"
>>> "python"[0]="o"
Traceback (most recent call last):
  File "", line 1, in 
    "python"[0]="o"
TypeError: "str" object does not support item assignment

4.tuple

>>> a = (1,2,3)
>>> a[0] = "1"
Traceback (most recent call last):
  File "", line 1, in 
    a[0] = "1"
TypeError: "tuple" object does not support item assignment
>>> b = [1,2,3]
>>> b.append(4)
>>> print(b)
[1, 2, 3, 4]
>>> c = (1,2,3)
>>> c.append(4)
Traceback (most recent call last):
  File "", line 1, in 
    c.append(4)
AttributeError: "tuple" object has no attribute "append"

>>> a = (1,2,3,[1,2,4])
>>> a[2]
3
>>> a[3]
[1, 2, 4]
>>> a[3][2]
4
>>> a = (1,2,3,[1,2,["a","b","c"]])
>>> a[3][2][1]
"b"
>>> a = (1,2,3,[1,2,4])
>>> a[2] = "3"
Traceback (most recent call last):
  File "", line 1, in 
    a[2] = "3"
TypeError: "tuple" object does not support item assignment
>>> a[3][2] = "4"
>>> print(a)    //元组内的列表可变
(1, 2, 3, [1, 2, "4"])

1.算数运算符：+，-，* ，/，//，%，**

>>> "hello"+"world"
"helloworld"
>>> [1,2,3]*3
[1, 2, 3, 1, 2, 3, 1, 2, 3]
>>> 3-1
2
>>> 3/2
1.5
>>> 3//2   //整除
1
>>> 5%2   //求余
1
>>> 2**2   //求N次方
4
>>> 2**5
32

2.赋值运算符：=，+=，-=，*=，/=，%=，**=，//=

>>> c = 1
>>> c = c+1
>>> print(c)
2
>>> c+=1
>>> print(c)
3
>>> c-=1
>>> print(c)
2
>>> c++   //python中没有自增和自减运算符
SyntaxError: invalid syntax
>>> c--
SyntaxError: invalid syntax
>>> b=2
>>> a=3
>>> b+=a
>>> print(b)
5
>>> b-=a
>>> print(b)
2
>>> b*=a
>>> print(b)
6

3.比较（关系）运算符：==，！=，>，<，>=，<=

>>> 1==1
True
>>> 1>1
False
>>> 1>=1
True
>>> a>=b
Traceback (most recent call last):
  File "", line 1, in 
    a>=b
NameError: name "a" is not defined
>>> a=1
>>> b=2
>>> a!=b
True
>>> b=1
>>> b+=b>=1    //b=b+True
>>> print(b)
2
>>> print(b>=1)     
True

>>> 1>1
False
>>> 2>3
False
>>> "a">"b"
False
>>> ord("a")
97
>>> ord("b")
98
>>> "abc"<"abd"   //实际上是a和a比，b和b比，c和d比
True
>>> ord("abc")
Traceback (most recent call last):
  File "", line 1, in 
    ord("abc")
TypeError: ord() expected a character, but string of length 3 found
>>> ord("c")
99
>>> ord("d")
100
>>> [1,2,3]<[2,3,4]
True
>>> (1,2,3)<(1,3,2)
True

4.逻辑运算符：and，or，not

>>> True and True
True
>>> True and False
False
>>> True or False
True
>>> False or False
False
>>> not False
True
>>> not True
False
>>> not not True
True

0 被认为是False，非0 表示True

>>> 1 and 1
1
>>> "a" and "b"
"b"
>>> "a" or "b"
"a"
>>> not "a"
False
>>> a = True
>>> b = False
>>> a or b
True
>>> b and a
False

空字符串 False

>>> not 0.1
False
>>> not ""
True
>>> not "0"
False

空的列表 False

>>> not []
True
>>> not [1,2]
False
>>> [1] or []
[1]
>>> [] or [1]
[1]
>>> "a" and "b"
"b"
>>> "" and "b"
""
>>> 1 and 0
0
>>> 0 and 1
0
>>> 1 and 2
2
>>> 2 and 1
1
>>> 0 or 1
1
>>> 1 or 0
1
>>> 1 or 2
1

5.成员运算符：in，not in

>>> a = 1
>>> a in [1,2,3,4,5]
True
>>> b = 6
>>> b in [1,2,3,4,5]
False
>>> b not in [1,2,3,4,5]
True
>>> b = "h"
>>> b in "hello"
True
>>> b not in (1,2,3,4,5)
True
>>> b not in {1,2,3,4,5}
True
>>> b = "a"
>>> b in {"c":1}
False
>>> b = 1
>>> b in {"c":1}
False
>>> b = "c"
>>> b in {"c":1}   //字典里面根据key返回
True

6.身份运算符：is，is not

对象的三个特征：id、value、type，判断id用“is”，判断value用“==”，判断type用“isinstance”

>>> a = 1
>>> b = 1
>>> a is b
True
>>> a="hello"
>>> b="world"
>>> a is b
False
>>> c="hello"
>>> a is c
True
>>> a=1
>>> b=2
>>> a==b
False
>>> a=1
>>> b=1
>>> a is b
True
>>> a==b
True
>>> a=1
>>> b=1.0
>>> a==b
True
>>> a is b   //is不是比较值相等，比较的是两个变量的身份（内存地址）是否相等
False
>>> id(a)
4374928384
>>> id(b)
4376239272


>>> a={1,2,3}
>>> b={2,1,3}
>>> a==b    //集合是无序的
True
>>> a is b
False
>>> id(a)
4433997384
>>> id(b)
4433996488

>>> c=(1,2,3)
>>> d=(2,1,3)
>>> c==d    //元组是序列，是有序的
False
>>> c is d
False

>>> a=1
>>> b=2
>>> a==b
False
>>> a is b
False

>>> a = "hello"
>>> type(a) == int
False
>>> type(a) == str
True
>>> isinstance(a,str)   //isinstance是判断变量类型的函数
True
>>> isinstance(a,int)
False
>>> isinstance(a,(int,str,float))
True
>>> isinstance(a,(int,float))
False

7.位运算符：（==把数字当作二进制数进行运算==）

&按位与

|按位或

^按位异或

~按位取反

<<左移动

>>右移动

按位与运算，每一个二进制数位进行对比，两个都为1，则得到1，只要有一个为0，就得到0

>>> a = 2
>>> b = 3
>>> a & b
2

按位或运算，每一个二进制数位进行对比，只要有一个为1，就得到1，两个都为0，则得到0

>>> a = 2
>>> b = 3
>>> a | b
3