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Python Basics (values, variables, operators)

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Python Basics （ value , Variable , Operator ）

pycharm operation

function ：ctrl + f10

Run the current file ：ctrl +shift + f10

One 、Python Output function in

print() function

It can output numbers
Can be a string
The output can be an expression with an operator

print() The function can output the content to

Monitor

file

# Output to a file , Be careful ：1, The specified drive letter exists , Use file=fp
fp = open('D:/text.txt','a+')
print('hello world',file=fp)
fp.close()

Create files if they don't exist , If it exists, it will be appended after the contents of the file

print() The output form of the function

Line break
- Execute the above again , Will be in text.txt Enter a new line in the file hello world

Don't wrap

# No line feed output ,
print('hello','world','python')

Two , Escape character and original character

What is an escape character ？
- It's a backslash + You want to implement the escape function
Why do you need escape characters ？
- When the string contains a backslash , When there are special-purpose characters such as single quotation marks and double quotation marks , These characters must be escaped with a backslash （ Convert a meaning ）
  - The backslash \
  - Single quotation marks ’
  - Double quotes "
- When a string contains a newline , enter , Special characters that cannot be directly represented, such as horizontal tabs or backspace, are , You can also use escape characters
  - Line break \n
  - enter \r
  - Horizontal tabs \t
  - Backspace \b

print('hello \n world')

Output ：

hello
world

Line feed

print('hello\nworld')
print('hello\tworld')
print('helloooo\tworld')

hello
world
hello world
helloooo world

One \t There are three spaces , the second \t There are four spaces , because hello Of o Has occupied a position , A tab stop is the size of four spaces , the second helloooo just 8 Characters , No tab stops

print('hello\rworld')

Output results ：

world

Because output hello after \r enter , It goes back to the beginning , And then put hello To kill

print('hello\bworld')

Output ：

hellworld

o be without , Because of backspace \b It's gone

print('http:\\\\www.baidu.com')

Output URL ：

http:\\www.baidu.com

Output quoted content ：

print(' The teacher said ：\' Hello everyone \'')

Output ：

 The teacher said ：' Hello everyone '

Original character , You don't want escape characters in strings to work , Use the original character , Is to add... Before the string r, or R

print(r'hello\nworld')

Output ：

hello\nworld

matters needing attention ： The last character is not a backslash

Such as ：

print(r'hello\nworld\')

Will report a mistake

however , It could be two

print(r'hello\nworld\\')

Output ：

hello\nworld\\

3、 ... and , Binary and character encoding

Recommended utf-8

Four 、Python Identifiers and reserved words in

The rules ：

Letter , Numbers , Underline
Cannot start with a number
It can't be reserved
Case sensitive

View reserved words ：

import keyword
print(keyword.kwlist)

Output results ：

[‘False’, ‘None’, ‘True’, ‘peg_parser’, ‘and’, ‘as’, ‘assert’, ‘async’, ‘await’, ‘break’, ‘class’, ‘continue’, ‘def’, ‘del’, ‘elif’, ‘else’, ‘except’, ‘finally’, ‘for’, ‘from’, ‘global’, ‘if’, ‘import’, ‘in’, ‘is’, ‘lambda’, ‘nonlocal’, ‘not’, ‘or’, ‘pass’, ‘raise’, ‘return’, ‘try’, ‘while’, ‘with’, ‘yield’]

5、 ... and , Definition and use of variables

A variable is a labeled box in memory

name = ' cheer up '
print(name)
print(' identification ',id(name))
print(' type ',type(name))
print(' value ',name)

Output ：

 cheer up
identification 2122703801680 # Memory address 
type <class 'str'> # type 
value cheer up

Multiple assignment of variables
```
nam = 'men'
nam = 'chu'
print(nam)
```
Output ：
```
chu
```
nam The direction of this variable will change from the original to the new space

6、 ... and ,Python Common data types in

Common data types
- Integer types int 98
- Floating point type float 3.14159
- Boolean type bool True False （ Start with a capital ）
- String type str ‘ Life is too short , I use Python’

Integer types

Can represent a positive number , negative , And zero
- ```
n1 = 90
n2 = -12
n3 = 0
print(n1,type(n1))
print(n2,type(n2))
print(n3,type(n3))
```
  Output ：
  90 <class ‘int’>
  -12 <class ‘int’>
  0 <class ‘int’>

Integers can be expressed as binary , Decimal system , octal , Hexadecimal

print(' Decimal system ',118)
print(' Binary system ',0b10110101) # Binary system 0b start 
print(' octal ',0o176) # octal 0o start 
print(' Hexadecimal ',0xEFA) # Hexadecimal 0x start

Output ：

Decimal system 118
Binary system 181
octal 126
Hexadecimal 3834

Floating point type

Integral part and decimal part make up

n4 = 3.14159
print(n4,type(n4))

Output ：

3.14159 <class ‘float’>

Floating point number storage is inaccurate
```
n5 = 1.1
n6 = 2.2
print(n5 + n6)
```
Output ：
3.3000000000000003
This is caused by the binary storage of the computer , But it doesn't have to be a mistake
```
n5 = 1.1
n6 = 2.1
print(n5 + n6)
n7 = 1.2
print(n6 + n7)
```
Output ：
3.2
3.3

terms of settlement

The import module decimal

from decimal import Decimal
print(Decimal('1.1') + Decimal('2.2'))

Output ：

3.3

Must have quotation marks ：

from decimal import Decimal
print(Decimal(1.1) + Decimal(2.2))

Output ：

3.300000000000000266453525910

Boolean type

Used to indicate true or false values

True,False

Boolean values can be converted to integers

True 1
False 0

f1 = True
f2 = False
print(f1,type(f1))
print(f2,type(f2))

Output

True <class ‘bool’>
False <class ‘bool’>

transformation ：

f1 = True
f2 = False
print(f1 + 1) #1+1 As the result of the 2,True Express 1
print(f2 + 1) #0+1 As the result of the 1,False Express 0

Output ：

2
1

String type

Strings are also called immutable character sequences
You can use single quotes ’’, Double quotes "", Three quotes ’’’ ‘’' or “”" “”" To define
The string defined by single quotation marks and double quotation marks must be on one line
A string defined by three quotation marks can be distributed on multiple consecutive lines

str1 = ' Life is too short , I use python'
str2 = " Life is too short , I use python"
str3 = ''' Life is too short , I use python'''
str4 = """ Life is too short , I use python"""
print(str1,type(str1))
print(str2,type(str2))
print(str3,type(str3))
print(str4,type(str4))

Output results ：

 Life is too short , I use python <class 'str'>
Life is too short , I use python <class 'str'>
Life is too short ,
I use python <class 'str'>
Life is too short ,
I use python <class 'str'>

Only those with three quotation marks can have multiple lines , And the output result is multi line

Type conversion str() Function and int() function

str() function

name = ' Zhang San '
age = 20
print(type(name),type(age))
# print(' My name is '+name+ ' This year, '+age+' year ') #str The type and int Type , Report errors , To type conversion 
print(' My name is '+name+ ' This year, '+str(age)+' year ') #str() Function performs type conversion , Turn into str type

<class ‘str’> <class ‘int’>
My name is Zhang San. This year 20 year

Other types are converted to str type
```
a = 10
b = 198.8
c = False
print(str(a),str(b),str(c))
```
Output ：
10 198.8 False
Type after conversion

print(str(a),str(b),str(c),type(str(a)), type(str(b)), type(str(c)))

Output ：

10 198.8 False <class ‘str’> <class ‘str’> <class ‘str’>

It can be seen that the type has changed

int() function

s1 = '128'
f1 = 98.7
s2 = '78.77'
ff = True
s3 = 'hello'
print(int(s1),type(int(s1))) # take str Turn into int type , String to numeric string 
print(int(f1),type(int(f1))) # take float Turn into int type , Will intercept the integer part , Omit the decimal part 
# print(int(s2),type(int(s2))) # take str To int Report errors , Because the string is a decimal string 
print(int(ff),type(int(ff))) # Converts a Boolean value to int,True Turn into 1,False Corresponding to 0
# print(int(s3),type(int(s3))) # take str To int Report errors , The string must be a number , And is an integer

Output ：

128 <class ‘int’>
98 <class ‘int’>
1 <class ‘int’>

Type conversion _float() function

s1 = '128.98'
s2 = '76'
ff = True
fb = False
s3 = 'hello'
i = 98
print(float(s1),type(float(s1)))
print(float(s2),type(float(s2)))
print(float(ff),type(float(ff))) # Into the float type ,True Corresponding 1.0
print(float(fb),type(float(fb))) # Into the float type ,False Corresponding 0.0
# print(float(s3),type(float(s3))) # Report errors , Not a numeric string 
print(float(i),type(float(i))) # Into the float type , And add a decimal part

Output ：

128.98 <class 'float'>
76.0 <class 'float'>
1.0 <class 'float'>
0.0 <class 'float'>
98.0 <class 'float'>

int float str They can be conditionally converted to each other , function int() str() float()

python The comments in

Single-line comments # start , Until the end of the line feed

Multiline comment Code between a pair of three quotation marks （ Three single quotes and three double quotes are OK ）

Comments on Chinese coding statement ： Add a Chinese statement note at the beginning of the document , Used to specify the encoding format of the source file

How to operate ： Create a new python file , The first line at the beginning of the file , No space , add

#coding:gbk

It can also be utf-8

Then you will know that this is an annotation file , Open in Notepad , When saving as, the format is ANSI

6、 ... and ,python The input function in the input()

effect ： Receive input from the user

return type ： The type of input value is str

Value storage ： Use = Store the entered value

a = input(' Please enter an addend ：')
b = input(' Please enter another addend ：')
print(a + b)

Output ：

 Please enter an addend ：10
Please enter another addend ：20
1020

Prove that the type of input changes str 了 , Then it becomes a concatenated string

Type conversion
- If you need integer and floating point types , You need to str Type through int() Function or float() Function for type conversion
  Example ：
```
a = int(input(' Please enter an addend ：'))
b = int(input(' Please enter another addend ：'))
print(a + b)
```
  Output ：
```
 Please enter an addend ：10
Please enter another addend ：20
30
```
  In this way, you can output , Get the output result conversion again, and then operate

7、 ... and , Operator

Common operators ： Arithmetic operator （ Standard arithmetic operators , The remainder operator , Power operator ）, Assignment operator , Comparison operator , Boolean operator , An operator

Arithmetic operator

Addition operation
- ```
print(1 + 1) # Addition operation
```
  Output ：
  2
Subtraction
- ```
print(2 - 1) # Subtraction 
```
  Output ：
  1
Multiplication
- ```
print(2*4) # Multiplication 
```
  Output ：
  8
Division operations
- ```
print(2/4) # Division operations 
```
  Output
  0.5

Division operation

print(11//2) # Take business , Division operation

Output ：

Take over operations
- ```
print(11%2) # Take over operations 
```
  Output ：
  1
Power operation
- ```
print(2**3) # Power operation 
```
  Output ：
  8 namely ： seek 2 Of 3 The next power
The special case of division
- ```
print(-9//4)
print(9//-4)
print(17//-4)
print(11//2)
```
  Output ：
  -3
  -3
  -5
  5
  reason ： One is one minus one. , Rounding down

Special case of remainder

print(9%-4) # The formula ： remainder = Divisor - Divisor * merchant 9-(-4)*(-3) = -3
print(17%-4)

Output ：

-3
-3

The quotient here should be the quotient of division

Assignment operator

Execution order ： Right To the left

Support chain assignment ： a=b=c=20

Support parameter assignment ： += -= *= /= //= %=

Support series unpacking assignment ：a,b,c=20,30,40

a=b=c=20
print(a)
print(b)
print(c)

Output ：

20
20
20

Chain assignment essence , Multiple variables point to a space at the same time

a = 2
b = 3
a **= b
print(a)

Output ：8

a,b,c = 20,30,40
print(a)
print(b)
print(c)
d,e,f=6,'egg',3.14159
print(d)
print(e)
print(f)

Output ：

Be careful ： The number of left and right variables should be consistent with the number of values

print("---------- Exchange the values of two variables --------")
a,b=10,20
print(' Before the exchange ：',a,b)
# In exchange for 
a,b=b,a
print(' After the exchange ：',a,b)

Output ：

 Before the exchange ： 10 20
After the exchange ： 20 10

Be careful ： effect , Do not use the third variable to transfer , Realize the exchange of values , Can compare other languages

Comparison operator

Size the result of a variable or expression 、 Comparison of true and false
type

>,<,>=,<=,!=
== # object value Comparison 
is, is not # Object's id Comparison

print('--------- Comparison operator ----------')
a,b=10,20
print('a>b Do you ？',a>b)
print('a<b Do you ？',a<b)
print('a>=b Do you ？',a>=b)
print('a<=b Do you ？',a<=b)
print('a==b Do you ？',a==b)
print('a!=b Do you ？',a!=b)

Output ：

--------- Comparison operator ----------
a>b Do you ？ False
a<b Do you ？ True
a>=b Do you ？ False
a<=b Do you ？ True
a==b Do you ？ False
a!=b Do you ？ True

Be careful ：

One = It's called the assignment operator , == It's called a comparison operator

A variable consists of three parts , identification , type , value

== Compare values , For comparison marks is, is not

A special case ：

a = 10
b = 10
print(a==b)
print(a is b )
print(a is not b)
list1=[11,22,33,44]
list2=[11,22,33,44]
print(list1==list2)
print(list1 is list2)
print(list1 is not list2)

Output ：

True
True
False
True
False
True

prove ： When it's a number , actually a,b All point to the same space id

and list Type will not

Boolean operator

For operations between Boolean values

and or not in not in

a,b=1,2
print(a==1 and b==2) #True True and True --->True
print(a==1 and b<2) #False True and False --->False
print(a!=1 and b == 2) #False False and True ---> False
print(a!=1 and b!=2) #False False and False --->False

Output ：

True
False
False
False

a,b=1,2
print(a==1 or b==2) #True True or True --->True
print(a==1 or b<2) #True True or False --->True
print(a!=1 or b == 2) #True False or True ---> True
print(a!=1 or b!=2) #False False or False --->False

Output ：

True
True
True
False

print('-----not Yes bool Type operand inversion ----')
f = True
ff = False
print(not f)
print(not ff)

Output ：

-----not Yes bool Type operand inversion ----
False
True

print('-----in And not in --------')
s = 'helloworld'
print('w' in s)
print('k' in s)
print('w' not in s)
print('k' not in s)

Output ：

-----in And not in --------
True
False
False
True

The list is waiting to be studied

An operator

 Bit and & The corresponding digits are 1, As a result, the number is 1, Otherwise 0
Bit or | The corresponding digits are 0, As a result, the number is 0, Otherwise 1
Left shift operator << High overflow discard , Low complement 0
Shift right operator >> Low overflow discard , High compensation 0

Operator priority

 First of all ： **
second ： *,/,//,%
Third ：+,-
These are arithmetic operators , Priority first
Fourth ： << >>
The fifth ：&
The sixth : |
Bit operator priority second echelon
The seventh ： >,<,>=,<=,==,!=
The third echelon of comparison operators
The eighth ：and
The ninth ：or
Fourth echelon ： Boolean operator
Last ：= Assignment operator