python学习笔记3---变量与运算符-白红宇

python学习笔记3---变量与运算符

阅读量：5792 次

发布时间：2019-06-18

本文共 5835 字，大约阅读时间需要 19 分钟。

什么是变量

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]

变量名的命名规则

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 = 1SyntaxError: invalid syntax>>> if = 2SyntaxError: invalid syntax>>> import = 3SyntaxError: 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}

值类型与引用类型

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'])

运算符

1.算数运算符：+，-，* ，/，//，%，**

>>> 'hello'+'world''helloworld'>>> [1,2,3]*3[1, 2, 3, 1, 2, 3, 1, 2, 3]>>> 3-12>>> 3/21.5>>> 3//2   //整除1>>> 5%2   //求余1>>> 2**2   //求N次方4>>> 2**532

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==1True>>> 1>1False>>> 1>=1True>>> a>=bTraceback (most recent call last):  File "
    
     ", line 1, in 
     
          a>=bNameError: name 'a' is not defined>>> a=1>>> b=2>>> a!=bTrue>>> b=1>>> b+=b>=1    //b=b+True>>> print(b)2>>> print(b>=1)     True>>> 1>1False>>> 2>3False>>> '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 TrueTrue>>> True and FalseFalse>>> True or FalseTrue>>> False or FalseFalse>>> not FalseTrue>>> not TrueFalse>>> not not TrueTrue

0 被认为是False，非0 表示True

>>> 1 and 11>>> 'a' and 'b''b'>>> 'a' or 'b''a'>>> not 'a'False>>> a = True>>> b = False>>> a or bTrue>>> b and aFalse

空字符串 False

>>> not 0.1False>>> 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 00>>> 0 and 10>>> 1 and 22>>> 2 and 11>>> 0 or 11>>> 1 or 01>>> 1 or 21

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 bTrue>>> a='hello'>>> b='world'>>> a is bFalse>>> c='hello'>>> a is cTrue>>> a=1>>> b=2>>> a==bFalse>>> a=1>>> b=1>>> a is bTrue>>> a==bTrue>>> a=1>>> b=1.0>>> a==bTrue>>> 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 bFalse>>> 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 dFalse>>> a=1>>> b=2>>> a==bFalse>>> a is bFalse>>> a = 'hello'>>> type(a) == intFalse>>> type(a) == strTrue>>> 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==）