Python basic knowledge collection

1. list

Lists are the most commonly used Python data type , It can appear as a comma separated value in square brackets .

The data items of a list do not need to have the same type

Create a list , Just enclose the different data items separated by commas in square brackets

Code understanding ： Index of the list , Addition, deletion and modification of list data , And the use of list derivation

# list 

all_in_list=[0.3,'hello','True']

print(' The list data is ：',end='')

print(all_in_list)
# The forward index starts from 

res=all_in_list[0]

print('all_in_list[0] The value of is ：',end='')

print(res)

# Reverse all slave -1 Start 

res=all_in_list[-2]

print('all_in_list[-2] The value of is ：',end='')

print(res)

# Slice of the list , Left closed right away 

res=all_in_list[0:2]

print('all_in_list[0:2] The slice value of is ：',end='')

print(res)
# The new element in the end 

all_in_list.append('hello world')

# Insert the element before the specified position , Become an element in the specified position 

all_in_list.insert(0,'pre-hello')

print(' Add the new element as ：',end='')

print(all_in_list)
# Remove elements 

all_in_list.remove('hello world')

# Delete the first two elements 

del all_in_list[:2]

print(' After deleting the element, it is ：',end='')

print(all_in_list)
# Modify element values 

all_in_list[0]=100

print(' Modify the element to ：',end='')

print(all_in_list)
# for loop 

x=[]

for i in range(10):

x.append(i)
print('for After adding the element in a loop, it is ：',end='')

print(x)
# List derivation 

b=[i for i in range(1,11)]

c=[i**2 for i in range(1,11)]

d=[i**2 for i in range(1,11) if i%2==0]

print(' The value of each list derivation is ：',end='')

print(b)

print(c)

print(d)
# practice 1： Find the area of curved trapezoid 

import math

n=10000

width=2*math.pi/n

# Method 1 ： utilize for Loop through the core data structure 

x=[]

y=[]

for i in range(n):

x.append(i*width)

for i in x:

y.append(abs(math.sin(i)))

S=sum(y)*width

print(' Method the area of a curved trapezoid is ：',end='')

print(S)

# Method 2 ： Use list derivation to build the core data structure 

s=[abs(math.sin(i*width))*width for i in range(n)]

print(' Methods the area of the trapezoid with two curved edges is ：',end='')

print(sum(s))

Code run results

2. The operator

Python Languages often use the following types of operators : Arithmetic operator 、 Compare relational operators 、 Assignment operator 、 Logical operators

Code understanding : Logical controller 、 Inclusion of strings in、 Conditional statements 、 Exchange value operation of bubble sort

# Python Logical controller 

res=1<2

print('1<2 The correctness of the ：',end='')

print(res)

res=1<2<3

print('1<2<3 The correctness of the ：',end='')

print(res)

res='Name'=='name'

print("'Name'=='name' The correctness of the ：",end='')

print(res)

res='M' in 'magic'

print("'M' stay 'magic' Correctness in ：",end='')

print(res)
# Conditional decision statement 

if 1<2:

print(' If 1<2 correct , The output ：', end='')

print('1.hello')
if 1<0:

print(' If 1<0 correct , The output ：', end='')

print('2.hello')

else:

print(' If 1<0 Incorrect , The output ：', end='')

print('2.world')
if 1<0:

print(' If 1<0 correct , The output ：', end='')

print('3.hello')

elif 2<1:

print(' If 1<0 Incorrect , but 2<1 correct , The output ：', end='')

print('3.world')

else:

print(' If 1<0 Incorrect , And 2<1 It's not true either , The output ：', end='')

print('3.hehe')
# Bubble sort 

x=[1,2,6,0.3,2,0.5,-1,2.4]

print(' The value before bubble sorting is ：', end='')

print(x)

n=len(x)

for i in range(n):

for j in range(i):

if x[j]>x[i]:

# Exchange value 

x[i],x[j]=x[j],x[i]
print(' The value after bubble sorting is ：', end='')

print(x)

Code run results

3. Dictionaries

Dictionary is another variable container model , And can store any type of object .

Each key value of the dictionary key:value Yes, with a colon : Division , Comma between each key value pair , Division , The whole dictionary is enclosed in curly brackets {} in , The format is as follows ：d = {key1 : value1, key2 : value2 }

Code understanding ： According to the dictionary index value , Modified value , add value , Delete value , And dictionary derivation to generate dictionary

# Dictionaries 

dic={'h':'hello',0.5:[0.2,0.3],'w':'world'}

print(' The dictionary data is ：',end='')

print(dic)
# According to the key index value 

res=dic['h']

print("dic['h'] The corresponding value is ：",end='')

print(res)

# Modify an element according to the key 

dic['h']=100

print(" The revised dictionary is ：",end='')

print(dic)

# Add an element 

dic['hw']='hello world'

print(" The dictionary after adding a key value is ：",end='')

print(dic)

# Add multiple elements at the same time 

dic.update({1:2,3:4})

print(" The dictionary after adding multiple key values is ：",end='')

print(dic)

# Delete an element according to the key 

del dic['h']

print(" The dictionary after deleting the key value is ：",end='')

print(dic)
# Dictionary derivation 

a={i:i**2 for i in range(10)}

print(" The dictionary generated by dictionary derivation is ：",end='')

print(a)

Code run results

4. character string

The string is Python The most commonly used data type in . We can use quotation marks ( ' or " ) To create a string .

Creating a string is simple , Just assign a value to the variable .

Code understanding ： There are two ways to generate strings with and without line breaks 、 Index of a string , Basic operation of splicing 、 String segmentation 、 All strings are lowercase

# character string , There is no difference between a double quoted string and a single quoted string 

string="My name"

print(' The generated string is ：'+string)

# Three quotation marks can be used for line breaking 

string='''My
name

'''

print(' The generated string with newline is ：'+string)

string='My name'

print(' Single quotation marks and double quotation marks generate the same string , It's for ：'+string)

print(string)

# The first element of the index string 

print(' The first element of the string is ：'+string[0])

# The first two elements of the index string 

print(' The first two elements of the string are ：'+string[:2])

# Repeat the string twice 

print(' Repeat the string twice as ：'+string*2)

# String concatenation 

print(' The splicing string is ：',end='')

print(string+' is xxx')
# Split string , Divide by commas , The return result is a list 

res=string.split(sep=',')

print(' The string is divided according to the comma and then is ：',end='')

print(1,string)

# Change all string letters to lowercase 

res=string.lower()

print(' Change all string letters to lowercase ：',end='')

print(res)

Code run results

5. file

Code understanding

# File operations 

f=open("Walden.txt","r")

# Read file contents 

txt=f.read()

print(txt)

# Before reading the contents of the file 100 That's ok 

txt=f.read(100)

print(txt)

f.close()

# Read the file line by line , And return to the list 

f=open("Walden.txt","r")

txt_lines=f.readlines()

print(txt_lines)

f.close()
# practice 3： Read the words in the novel 

import re

f=open("Walden.txt","r")

# The data type read in is string 

txt=f.read()

f.close()

# Change the characters in the string to lowercase 

txt=txt.lower()

# Remove the punctuation marks from the novel 

txt=re.sub('[,.?:“\’!-]','',txt)

# Word segmentation 

words=txt.split()

word_sq={}

for i in words:

if i not in word_sq.keys():

word_sq[i]=1

else:

word_sq[i]+=1

# Sort 

res=sorted(word_sq.items(),key=lambda x:x[1],reverse=True)

print(res)

6. function

You can define a function that you want to function , Here are the simple rules ：

• Function code block to def Key words start with , Followed by function identifier name and parentheses ().
• Any arguments and arguments passed in must be placed between parentheses . Parentheses can be used to define parameters .
• The first line of the function optionally uses the document string — Used to store function descriptions .
• Function contents start with a colon , And indent .
• return [ expression ] End function , Optionally return a value to the caller . Without expression return It's equivalent to returning to None.

Code understanding

# Function customization 

# The first way 

def Sum(x,y):

return x+y
# The second way , Simple customization of functions 

# return x The square of 

y=lambda x:x**2

# return x The first element of 

y1=lambda x:x[1]
res=Sum(1,2)

print(res)

res=y(10)

print(res)

res=y1(['hello',0])

print(res)
# practice 4： User defined function for finding the even number of sequences 

def su(x):

z=0

for i in x:

if i%2==0:

z+=1

return z

res=su([1,2,3,4,5,6])

print(res)

7. object-oriented

Some basic features of object orientation ：

• class (Class): A collection of objects with the same properties and methods . It defines the properties and methods that are common to each object in the collection . Object is an instance of a class .
• Class variables ： Class variables are common to the entire instantiated object . Class variables are defined in the class and outside the function body . Class variables are usually not used as instance variables .
• Data member ： Class variable or instance variable , Used to process data related to classes and their instance objects .
• Method rewriting ： If the method inherited from the parent class does not meet the needs of the child class , It can be rewritten , This process is called method coverage （override）, Also known as method rewriting .
• local variable ： Variables defined in methods , Class that only works on the current instance .
• Instance variables ： In the declaration of a class , Properties are represented by variables . This variable is called instance variable , It is declared inside the class declaration but outside the other member methods of the class .
• Inherit ： That is, a derived class （derived class） Inherited base class （base class） Fields and methods for . Inheritance also allows the object of a derived class to be treated as a base class object . for example , There is such a design ： One Dog Object of type derived from Animal class , This is a simulation " It's a （is-a）" Relationship （ Case diagram ,Dog It's a Animal）.
• Instantiation ： Create an instance of a class , Class specific objects .
• Method ： Functions defined in class .
• object ： Data structure instance defined by class . Object contains two data members （ Class variables and instance variables ） And methods .

Code understanding

# The difference between method and function 

all_in_list=[2.5,'hello','world',3]

string='My name'

all_in_list.append('hehe')

# For string objects , It's not append Methodical 

#string.append('Y’)
res=string.split()

# split It's a method , Is a method only available for string objects 

#all_in_list.split()

print(all_in_list)

print(res)
# object-oriented 

class Human:

def __init__(self,ag=None,se=None):

# Attributes of a class 

self.age=ag

self.sex=se

# Class method 

def square(self,x):

return x**2
zhangfei=Human(ag=23,se=' male ')

res=zhangfei.square(10)

print(res)

res=zhangfei.age

print(res)

res=zhangfei.sex

print(res)

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