Summary of essential knowledge for learning Python

This article mainly introduces the relevant information that summarizes the necessary knowledge for learning Python. Friends who need it can refer to it

1. Variables
1. Variables
•refers to the variables during program execution. , a variable quantity;
• Defining a variable will be accompanied by three characteristics, namely memory ID, data type and variable value.
•Before running other languages, you must manually release the memory space of the program. However, the python interpreter has its own memory recycling mechanism. Once the python program finishes running, the memory space will be automatically released.

age=10
print(id(age),type(age),age)

2. Constant
•Refers to an immutable quantity during program execution ;
•Constants are generally defined in uppercase letters.

AGE=10
print(AGE)

3. Naming method of variables
•Camel case

AgeOfOldboy=72

• Underscore

age_of_oldboy=72

2. Interacting with the program
In ancient times, when we went to the bank to withdraw money, we needed a bank clerk waiting for us to enter our account and password to him. , and then he goes to verify it and after it is successful, we enter the withdrawal amount and tell him.
Proud modern people will provide customers with an ATM machine (that is, a computer), allowing the ATM machine to interact with the user, thus replacing manpower. However, the machine is dead, and we must write programs for it to run. This requires our programming language to have a mechanism that can interact with the user and receive user input data.

1.python3
•Python3 supports UTF-8 Chinese encoding by default. python2 needs to add # -*- coding:utf-8 -*- at the head of the code.
•Input in python3, no matter what type of value is entered, it will be saved as str (string) type

name=input('please enter the username: ')
print(id(name),type(name),name)

2.python2
•raw_input in python2 is the same as input in python3;

name=raw_input('please enter the username: ')
print(id(name),type(name),name)

•Input in python2, a value must be entered, and the value will be saved as what type it is.

name=input('please enter the username: ')
print(id(name),type(name),name)

3. Data type
1.int integer
•Generally used to define age, ID number, QQ number, grade, etc.

age=18
id=130530198805240011
qq=379048558
level=99

2.float floating point Type
•Generally used to define height, weight, salary, etc.

height=1.81
height=float(1.81)

3.str String type
•Generally used to define a person’s name, gender, status, etc.;
•General Strings are placed in single quotes, double quotes, or triple quotes.

name='egon'
sex='female'
age=18

•String splicing uses " "

name='egon'
sex='female'
age=18

print(name sex str(age))
Note: The age variable value here is 18, which is an integer and cannot be used as a character. For string concatenation, str(age) needs to be used to convert to string type.

•Use "*" for string splicing

name='egon'
print(name*10)

4.bool Boolean value type
•Only two values: True and False;
•Mostly used for judgment .

age=73
AGE=18
print(age < AGE)
print(age > AGE)

5. Conversion between various types
•Integer type——>Floating point type

a=18
print(float(a))

•Floating point type——>Integer type

a=1.81
print(int(a))

•Floating point type——>String type

a=1.81
print(str(a))

•Integer type——>String type

a=18
print(str(a) )

4. Array type
1. List []
•List in python is defined in [], and the elements are separated by "comma";

info=['egon','alex',18]
print(info[2])

•Elements can be any data type or array type;
•Character elements need to be enclosed in quotes, integers, floating point types, lists, etc. do not need quotes.

info=[13,18.1,'alex',['egon','tony']]
print(info[3][0])

2. Dictionary {}
•Dictionary in python, also called associative array, is defined in {}, and the elements inside are expressed in the project name: project content format, and "comma" is used between elements "separated;

info={'name':'egon','sex':'male',3:18}
print(info[3])

•The project content can be any data type, any array type;
•The string type in the project content needs to be enclosed in quotes, integer, floating point, Lists, etc. do not require quotes.

info={'姓名':'爱根','性别':'男','肌肉':['有','无']}
print(info['肌肉'][1])
 
info={'姓名':'爱根','性别':'男','肌肉':123}
print(info['肌肉'])
 
info={'姓名':'爱根','性别':'男','肌肉':18.1}
print(info['肌肉'])
 
info={'姓名':'爱根','性别':'男','肌肉':'无'}
print(info['肌肉'][1])

5. Formatted output
•my name is xxx, my age is xxx
•You need to use the placeholder %s

name=input('user_name>>: ')
age=input('user_age>>: ')
print('my name is %s, my age is %s' %(name,age))

6. Operators
1. Arithmetic operators
• - * /

print(5+5) #5加5等于10
print(5-5) #5减5等于0
print(5*5) #5乘5等于25
print(5/2) #5除以2等于2.5

• Find the integer part of the quotient // Find the remainder part of the quotient % power **

print(5//2) #5除以2商等于2余1，只取商2
print(5%2) #5除以2商等于2余1，只取余数1
print(3**2) #3的2次幂是3乘3等于9

2. Comparison operators
• > •Logical AND, all conditions must be met before the result is True;

•Logical or or, only one condition is met, the result is True;

•Logical NOT, the result is negated.

print(30 > 20)
print(30 < 20)
print(30 >= 30)
print(30 <= 30)
print(30 == 30)
print(30 != 40)

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