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Code examples of variables and operators in python
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This article brings you code examples about variables and operators in python. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.
What is a variable
Assume two lists to perform mathematical operations
>>> [1,2,3,4,5,6] [1,2,3] Traceback (most recent call last): File "<pyshell#0>", line 1, in <module> [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]
Naming rules for variable names
Variable names can only use letters, numbers, and underscores
>>> 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
System keywords, cannot be used in variable names Reserved keywords
>>> 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 "<pyshell#1>", line 1, in <module> type(1) TypeError: 'int' object is not callable >>> 1(1) Traceback (most recent call last): File "<pyshell#2>", line 1, in <module> 1(1) TypeError: 'int' object is not callable
Features of python dynamic language, not required when declaring Specify the variable type
>>> a = '1'
>>> a = 1
>>> a = (1,2,3)
>>> a = {1,2,3}
Value type and reference type
int, str, and tuple are value types (immutable), and list, set, and dict are reference types (can Change)
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 "<pyshell#8>", line 1, in <module> '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 "<pyshell#21>", line 1, in <module> 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 "<pyshell#26>", line 1, in <module> 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 "<pyshell#7>", line 1, in <module> 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'])
Operator
1. Arithmetic operators: ,-,*,/,//,%,**
>>> '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. Assignment operators: =, =,- =, *=, /=, %=, **=, //=
>>> 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. Comparison (relational) operators: ==, ! =, >, <, >=, <=
>>> 1==1
True
>>> 1>1
False
>>> 1>=1
True
>>> a>=b
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
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 "<pyshell#12>", line 1, in <module>
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)
True4. Logical operators: 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</p> <p>0 is considered False, non-0 means True </p> <pre class="brush:php;toolbar:false">>>> 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
Empty string False
>>> not 0.1 False >>> not '' True >>> not '0' False
Empty list 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.Member operator: 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.Identity operation Symbol: is, is not
Three characteristics of the object: id, value, type, use "is" to judge id, use "==" to judge value, and use "isinstance" to judge type
>>> 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. Bitwise operators: (== Operate numbers as binary numbers==)
&bitwise AND
|Bitwise OR
^Bitwise XOR
~Bitwise negation
<
>>Move right
Bitwise AND operation, compare each binary digit, two If both are 1, you get 1. As long as one of them is 0, you get 0
>>> a = 2 >>> b = 3 >>> a & b 2
| Variable | Convert to decimal | |||
|---|---|---|---|---|
| 1 | 0 | 2 | ||
| 1 | 1 | 3 | ||
| 0 | 2 |
| Convert to decimal |
||||
|---|---|---|---|---|
| 0 | 2 |
b |
||
| 1 | 3 |
Bitwise OR |
||
| 1 | 3 |
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