Python Basics (values, variables, operators)

Posted by ben2005 on Mon, 14 Feb 2022 15:08:49 +0100

Python Basics (values, variables, operators)

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1, Output functions in Python

print() function

You can output numbers
Can be a string
The output can be an expression containing an operator

The print() function can output the content to the destination

monitor

file

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

If the file does not exist, it will be created. If it exists, it will be appended after the file content

The output form of the print() function

Line feed
- Execute the above again and it will be displayed in text Txt file and enter hello world again

nowrap

#Output without line feed,
print('hello','world','python')

2, Escape character and original character

What is an escape character?
- Is the backslash + the first letter that you want to escape
Why do I need escape characters?
- When a string contains special-purpose characters such as backslash, single quotation mark and double quotation mark, these characters must be escaped (converted into a meaning) with backslash
  - Backslash\
  - Single quote '
  - Double quotation mark“
- Escape characters can also be used when the string contains special characters that cannot be directly represented, such as line feed, carriage return, horizontal tab or backspace
  - Wrap line \ n
  - Enter \ r
  - Horizontal tab \ t
  - Backspace \ b

print('hello \n world')

Output:

hello
world

There are line breaks

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

hello
world
hello world
helloooo world

One \ t is three spaces, and the second \ t is four spaces, because the o of hello has occupied a position, one tab stop is the size of four spaces, and the second helloooo is just 8 characters, not tab stops

print('hello\rworld')

Output result:

world

Because after outputting Hello, press enter, then you will return to the beginning and kill the hello

print('hello\bworld')

Output:

hellworld

o no, because the backspace \ b is 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. If you don't want the escape character in the string to work, use the original character, that is, add R or r before the string

print(r'hello\nworld')

Output:

hello\nworld

Note: the last character is not a backslash

For example:

print(r'hello\nworld\')

Will report an error

However, it can be two

print(r'hello\nworld\\')

Output:

hello\nworld\\

3, Binary and character encoding

utf-8 is recommended

4, Identifiers and reserved words in Python

Rules:

Letters, numbers, underscores
Cannot start with a number
Cannot be a reserved word
Strictly case sensitive

View reserved words:

import keyword
print(keyword.kwlist)

Output result:

['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, Definition and use of variables

A variable is a labeled box in memory

name = 'awesome'
print(name)
print('identification',id(name))
print('type',type(name))
print('value',name)

Output:

awesome
 Identification 2122703801680    #Memory address
 type <class 'str'>    #type
 It's worth Ollie

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

6, Common data types in Python

Common data types
- Integer type int 98
- Floating point type float 3.14159
- Boolean type bool True False (start with uppercase)
- String type str 'life is short, I use Python'

Integer type

Can represent positive numbers, negative numbers, 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 in binary, decimal, octal and hexadecimal

print('decimal system',118)
print('Binary',0b10110101)  #Starting with binary 0b
print('octal number system',0o176)      #Octal 0o    
print('hexadecimal',0xEFA)     #Starting with hex 0x

Output:

Decimal 118
Binary 181
Octal 126
Hex 3834

Floating point type

Integral part and decimal part

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

Output:

3.14159 <class 'float'>

Floating point storage imprecise
```
n5 = 1.1
n6 = 2.2
print(n5 + n6)
```
Output:

3.3000000000000003

This is caused by the binary storage of the computer, but it is not necessarily wrong
```
n5 = 1.1
n6 = 2.1
print(n5 + n6)
n7 = 1.2
print(n6 + n7)
```
Output:

3.2
3.3

terms of settlement

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)  #The result of 1 + 1 is 2, and True means 1
print(f2 + 1)  #The result of 0 + 1 is 1, and False indicates 0

Output:

2
1

String type

Strings are also called immutable character sequences
You can use single quotation marks' ', double quotation marks'', three quotation marks' ', or' '' '
The string defined by single quotation marks and double quotation marks must be on one line
The string defined by three quotation marks can be distributed on multiple consecutive lines

str1 = 'Life is short, I use it python'
str2 = "Life is short, I use it python"
str3 = '''Life is short,
I use python'''
str4 = """Life is short,
I use python"""

print(str1,type(str1))
print(str2,type(str2))
print(str3,type(str3))
print(str4,type(str4))

Output result:

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

Only three quotation marks can be used for multiple lines, and the output result is also multiple lines

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')   #When str type and int type are connected, an error is reported and type conversion is required
print('My name is'+name+ 'this year'+str(age)+'year') #str() function performs type conversion and converts it to str type

<class 'str'> <class 'int'>
My name is Zhang San. I'm 20 years old

Convert other types to str types
```
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))) #Convert str to int type and string to numeric string
print(int(f1),type(int(f1))) #Converting float to int type will intercept the integer part and round off the decimal part
# print(int(s2),type(int(s2))) #An error is reported when str is converted to int because the string is a decimal string
print(int(ff),type(int(ff))) #Convert Boolean value to int,True to 1, and False to 0
# print(int(s3),type(int(s3))) #If str is converted to int, an error is reported. The string must be a number and 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)))  #Converted to float type, True corresponds to 1.0
print(float(fb),type(float(fb)))  #Converted to float type, False corresponds to 0.0
# print(float(s3),type(float(s3)))  #Error, not a numeric string
print(float(i),type(float(i)))   #Converted to float type and added 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 can be conditionally converted to each other, and the function int() str() float()

Comments in python

A single line # begins with a comment and ends with a newline

Multi line comment code between a pair of three quotation marks (three single quotation marks and three double quotation marks are OK)

Chinese code declaration comments: add Chinese declaration comments at the beginning of the file to specify the coding format of the source file

How to operate: create a new python file, the first line at the beginning of the file, without spaces, plus

#coding:gbk

It can also be utf-8

Then you will know that this is a comment file. Open it with notepad and find that the format is ANSI when saving

6, Input function input() in python

Function: receive input from the user

Return value type: the type of the input value is str

Value storage: use = to 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 input type changes to str and then becomes a concatenated string

Type conversion
- If integer type and floating-point type are required, str type needs to be converted through int() function or float() function
  
  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 not only output, but also obtain the output result conversion and operation

7, Operator

Common operators: arithmetic operator (standard arithmetic operator, remainder operator, power operator), assignment operator, comparison operator, Boolean operator, bit operator

Arithmetic operator

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

Division operation

print(11//2) # quotient, integer division

Output:

Remainder operation
- ```
print(11%2)   #Remainder operation
```
  Output:
  
  1
exponentiation
- ```
print(2**3) #exponentiation 
```
  Output:
  
  8, that is, find the third power of 2
Special case of division
- ```
print(-9//4)
print(9//-4)
print(17//-4)
print(11//2)
```
  Output:
  
  -3
  -3
  -5
  5
  
  Reason: one positive and one negative, rounded down

Special case of remainder

print(9%-4)   #Formula: remainder = divisor divisor * quotient 9 - (- 4) * (- 3) = - 3
print(17%-4)

Output:

-3
-3

The quotient here should be the quotient of division

Assignment Operators

Execution order: right to 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

The essence of chain assignment is that 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:

Note: 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 exchange:',a,b)
#exchange
a,b=b,a
print('After exchange:',a,b)

Output:

Before exchange: 10 20
 After exchange: 20 10

Note: the function does not need the transfer of the third variable to realize the exchange of values, which can be compared with other languages

Comparison operator

Compare the results of variables or expressions in size, true and false
type

>,<,>=,<=,!=
==  #Comparison of object value
is, is not   #Comparison of object IDS

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

Output:

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

be careful:

A = is called the assignment operator and a = = is called the comparison operator

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

==The value is compared, and the comparison ID is, is not

exceptional 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

Proof: when it is a number, in fact, a and B point to the id of the same space

The list type does 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

List waiting study

Bitwise Operators

Bit and&  The corresponding digits are all 1, and the result digit is 1, otherwise it is 0
 Bit or | The corresponding digits are all 0, and the result digit is 0, otherwise it is 1
 Left shift operator<<  Discard the overflow at high level and make up 0 at low level
 Shift right operator>>  Low overflow discard, high make up 0

Operator priority

First: **

Second: *，/,//,%  

Third:+，-

The above are arithmetic operators, with priority first

Fourth: <<  >>

Fifth:&

sixth: |

Bitwise operator priority second echelon

Seventh: >,<,>=,<=,==,!=

Comparison operator third echelon

Eighth: and

Article 9: or

Fourth echelon: Boolean operator

last:=  Assignment Operators

Topics: Python Pycharm

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Python Basics (values, variables, operators)

Python Basics (values, variables, operators)

pycharm operation

1, Output functions in Python

2, Escape character and original character

3, Binary and character encoding

4, Identifiers and reserved words in Python

5, Definition and use of variables

6, Common data types in Python

Integer type

Floating point type

Boolean type

String type

Type conversion str() function and int() function

Type conversion_ float() function

Comments in python

6, Input function input() in python

7, Operator

Arithmetic operator

Assignment Operators

Comparison operator

Boolean operator

Bitwise Operators

Operator priority

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