Life is short, I learn Python

osc_ fhqvuif4 2021-01-20 12:38:18
life short learn python


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One 、 List Introduction

(1) What is a list

A list consists of a series of elements arranged in a specific order . These elements can be : Numbers 、 Letter 、 Strings and other types of data . stay Python in , In square brackets [ ] To represent a list , And use commas to separate the elements . Here's a simple example of a list :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
print(week_list)

If you use print() function , Python The internal representation of the list will be printed , Include brackets :

['Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

If you want to output every element in the list directly , Then you have to learn how to access the elements of the list .

(2) Access list elements

A list is an ordered set , So to access any element of the list , Just know where the element is in the list . To access list elements , You can indicate the name of the list , And then point out the index of the element , And put it in square brackets . for example , We want to get the Friday:

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
print(Week_list[4])

The above demonstrates the syntax for accessing list elements . When you ask for a list element , Python Returns only the value of the element , It doesn't include square brackets and quotation marks :

Friday

Of course , If you want to output each element in the list in turn , Can pass for Realization :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
for day in Week_list:
print(day)

adopt for Loop through the list , Output each element in turn :

Monday
Tuseday
Wednseday
Thursday
Friday
Saturday
Sunday

(3) Nested list

Nested lists create other lists in the list , for example :

list1 = [1, 2, 3]
list2 = [" Cao Cao ", " king of Wu in the Three Kingdoms Era ", " Liu bei "]
list = [list1, list2]
print(list)

Nested list means that the elements in the list are lists :

[[1, 2, 3], [' Cao Cao ', ' king of Wu in the Three Kingdoms Era ', ' Liu bei ']]

When you want to take an element in the list , such as “ Cao Cao ”, You need to learn how to use the index .

Two 、 Indexes 、 Slicing and list copying

(1) Indexes

The index value depends on the position of the element . stay Python in , The index of the first list element is 0 , instead of 1 . According to this simple way of counting , To access any element of the list , Can be reduced to 1 , And index the results . for example , To visit the 10 List elements , You can use the index 9.

For normal lists , When we want to visit its second 5 Element time :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
print(Week_list[4])

Program input :

Friday

For nested lists , When we want to access the elements , Like in the list above “ Cao Cao ”, In the following way :

list = [[1, 2, 3], [' Cao Cao ', ' king of Wu in the Three Kingdoms Era ', ' Liu bei ']]
print(list[1][0])

Program input :

 Cao Cao

among ,list[1][0] Index value in 1 Take the second list , Index value 0 It means to take the first element in the second list .

(2) section

The index provides access to a single list element , By slicing, you can handle the list of All or part of the elements . To create slices , You can specify the index of the first and last element to be used . The main thing is :Python Stop after reaching the element before the second index you specify .

1. for example : To output the first three elements in the list , Need to specify index 0~3 , This will output 0 、 1 and 2 The elements of .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
print(Week_list[0:3])

Slice the first three elements of the list :

['Monday', 'Tuseday', 'Wednseday']

2. You can also get any subset of the list , for example , If you want to extract the first 2~6 Elements , The starting index can be specified as 1 , And specify the termination index as 6 :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
print(Week_list[1:6])

Output Tuesday to Saturday :

['Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday']

3. If the first index is not specified , Python Will automatically start at the beginning of the list :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
print(Week_list[:6])

Output Monday to Saturday :

['Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday']

4. If you want to take the second 3 Elements to all elements at the end of the list , It can also be done in the following ways :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
print(Week_list[2:])

Go back to Wednesday to Sunday :

['Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

5. If you want to traverse parts of the list , Then you can for Use slices in the loop .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
for day in Week_list[:3]:
print(day)

Traverse the list 1 to 3 Elements , And the output :

Monday
Tuseday
Wednseday

(3) Copy list

In the development process , We often need to create new lists based on existing lists . Let's talk about Copy list How it works , And a situation where copying lists can be of great help .

To copy a list , You can create a slice that contains the entire list , The method is to omit both the starting index and the ending index [ : ] . This makes Python Creating one starts with the first element , A slice that ends at the last element , Copy the entire list .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list_copy1 = Week_list
Week_list_copy2 = Week_list[:]
print("Week_list:", id(Week_list))
print("Week_list_copy1:", id(Week_list_copy1))
print("Week_list_copy2:", id(Week_list_copy2))

We use and don't use [ : ] Two ways to copy , Get the list of id as follows :

Week_list: 298963473096
Week_list_copy1: 298963473096
Week_list_copy2: 298963418440

It can be seen from the results :Week_list_copy1 Same as original list Week_list Of id identical , explain Week_list_copy1 And the original list points to the same list ;Week_list_copy2 Same as original list Week_list Of id Different , Explain through [ : ] The way to copy , What you create is a new list .

3、 ... and 、 Number of list elements 、 Maximum 、 minimum value 、 transformation

stay Python in , Contains some built-in functions , It's easy to handle data like lists .

(1) Number of return elements

For normal lists , Returns the number of list elements :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
print(len(Week_list))

Program output :

7

For nested lists , Returns the number of lists in the list :

list = [[1, 2, 3], [' Cao Cao ', ' king of Wu in the Three Kingdoms Era ', ' Liu bei ']]
print(len(list))

Program output :

2

(2) Returns the maximum value of the element

max() Method returns the maximum value in the list element :

list1, list2 = ['Google', 'Baidu', 'Taobao'], [1, 2, 3]
print("list1 Maximum element value : ", max(list1))
print("list2 Maximum element value : ", max(list2))

The maximum value of the elements in the output list :

list1  Maximum element value : Taobao
list2  Maximum element value : 3

(3) Returns the minimum value of the element

min() Method returns the minimum value in the list element :

list1, list2 = ['Google', 'Baidu', 'Taobao'], [1, 2, 3]
print("list1 Minimum element value : ", min(list1))
print("list2 Minimum element value : ", min(list2))

The minimum value of the element in the output list :

list1  Minimum element value : Baidu
list2  Minimum element value : 1

(4) Converts a tuple to a list

list() Method to convert a tuple or string to a list :

str = "Hello Python"
list2 = list(str)
print(" List elements : ", list2)
Tuple = (123, 'Google', 'baidu', 'Taobao')
list1 = list(Tuple)
print(" List elements : ", list1)

The result of the procedure :

 List elements : ['H', 'e', 'l', 'l', 'o', ' ', 'P', 'y', 't', 'h', 'o', 'n']
List elements : [123, 'Google', 'baidu', 'Taobao']

notes : Tuples are very similar to lists , The difference is that the element value of a tuple cannot be modified , Tuples are in parentheses , The list is in square brackets .

Four 、 modify 、 Add and delete operations

(1) Modify list elements

The syntax for modifying list elements is similar to that for accessing list elements . To modify a list element , You can specify the list name and the index of the element to be modified , Then specify the new value of the element . For example, below , We'll take the first value in the list :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
Week_list[0] = " Monday "
print(Week_list)

Put... In the list Monday Change it to “ Monday ”:

[' Monday ', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

Of course , You can modify the values of any list element , Not just the value of the first element .

(2) Add elements at the end of the list

When you add a new element to the list , The easiest way is to use append() Add elements to the end of the list :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
Week_list.append(100)
print(Week_list)

take 100 Add to the end of the list :

['Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday', 100]

Use append() It's easy to create lists , for example , You can create an empty list first , Use a series of append() Statement to add elements . Let's create an empty list , Add elements to it :

Week_list = []
Week_list.append(1)
Week_list.append(2)
Week_list.append(3)
print(Week_list)

Program output :

[1, 2, 3]

(3) Insert elements in the list

Usage method insert() You can add new elements anywhere in the list . So , You need to specify the index and value of the new element :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday",
"Friday", "Saturday", "Sunday"]
Week_list.insert(0, 100)
print(Week_list)

Add... At the beginning of the list 100:

[100, 'Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

(4) Delete list elements

In data processing , We often need to remove one or more elements from the list .Python Provides 4 To delete elements from a list :

1. Use del Statement delete element . If you know where the element to delete is in the list , You can use del sentence .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
del Week_list[0]
print(Week_list)

Delete the first element in the list :

['Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

2. Usage method pop() Remove elements . Method pop() You can delete the elements at the end of the list , And allow you to continue to use it .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.pop()
print(Week_list)

Delete the last element in the list :

['Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday']

3. Delete elements anywhere in the list . actually , Use pop() You can also delete elements anywhere in the list , Just specify the index of the element to be deleted in brackets .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.pop(1)
print(Week_list)

Delete the second element in the list :

['Monday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

notes : If you're not sure you should use del Statement or pop() Method , Here's a simple criterion : If you want to remove an element from the list , And no longer use it in any way , Just use del sentence ; If you're going to be able to use it after deleting the element , Just use the method pop() .

4. Delete elements... Based on values . occasionally , You don't know where the value to delete from the list is . If you only know the value of the element to be deleted , Serviceable method remove() .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.remove("Monday")
print(Week_list)

Delete the elements in the list "Monday":

['Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

notes :remove() What is deleted is the first match of a value in the list

5、 ... and 、 List operators

(1) Splicing operators

stay Python,+ It can also be used as a splice operator , Put different lists together into one list :

list1 = [1, 2, 3]
list2 = [4, 5, 6]
print(list1 + list2)

Put together a list :

[1, 2, 3, 4, 5, 6]

(2) Copy operator

Use * The operator , You can easily copy list elements :

list = [1, 2, 3]
print(list * 2)

Copy all the elements in the list to 2 times :

[1, 2, 3, 1, 2, 3]

In the same way , It can be replicated many times .

6、 ... and 、 Sort the list

In the actual development process , Sometimes you want to keep the list elements in their original order , And sometimes you need to adjust the order . Python There are many ways to organize lists , You can choose according to the specific situation .

(1) Permanent sort sort()

sort() It makes it easier for you to sort the list , The elements in the list are sorted alphabetically :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.sort()
print(Week_list)

adopt sort() Sort , Is to permanently change the order of elements in the original list :

['Friday', 'Monday', 'Saturday', 'Sunday', 'Thursday', 'Tuseday', 'Wednseday']

When sorting in descending order , Just to sort() Method pass parameter reverse=True, As shown below :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.sort(reverse=True)
print(Week_list)

The descending sort result is as follows :

['Wednseday', 'Tuseday', 'Thursday', 'Sunday', 'Saturday', 'Monday', 'Friday']

(2) Temporary sorting sorted()

Sometimes I want to sort the list , If necessary, keep the order of the elements in the original list , You can use sorted() function .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list_sorted = sorted(Week_list)
print(Week_list_sorted)
print(Week_list)

Compare the sorted results :

['Friday', 'Monday', 'Saturday', 'Sunday', 'Thursday', 'Tuseday', 'Wednseday']
['Monday', 'Tuseday', 'Wednseday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

It can be seen that :sorted() It doesn't change the order of the elements in the original list .

7、 ... and 、 Other member functions

(1) Count the number of times an element appears count()

count() Method is used to count the number of times an element appears in the list . such as :

list = [1, 2, 2, 2, 3]
print(" In the list 2 The number of :", list.count(2))

Running results :

 In the list 2 The number of : 3

(2) Find the index position of the first match of a value in the list index()

index() Function to find the index position of the first match of a value from the list . For example, find out 2 The index of :

list = [1, 2, 2, 2, 3]
print(" In the list 2 First occurrence index :", list.index(2))

Running results :

 In the list 2 First occurrence index : 1

in addition , You can also specify a location in the list to start searching :

list = [1, 2, 2, 2, 3, 2, 3]
print(" Starting with the fifth element , In the list 2 First occurrence index :", list.index(2, 4))

From index value to 4 Start searching for , The result is :

 Starting with the fifth element , In the list 2 First occurrence index : 5

(3) Reverse list of elements reverse()

reverse() Function is used to reverse the list of elements . When we want to reverse the list of elements :

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.reverse()
print(Week_list)

After reversing the position of the elements in the list , The output is :

['Sunday', 'Saturday', 'Friday', 'Thursday', 'Wednseday', 'Tuseday', 'Monday']

(4) clear list clear()

clear() Function to clear the list , Be similar to del a[:].

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list.clear()
print (" After the list is cleared : ", Week_list)

After the list is cleared :

 After the list is cleared : []

(5) Copy list copy()

copy() Function for Copy list , Similar to the index mentioned above [:] The way it's copied .

Week_list = ["Monday", "Tuseday", "Wednseday", "Thursday", "Friday", "Saturday", "Sunday"]
Week_list_copy1 = Week_list.copy()
print("Week_list:", id(Week_list))
print("Week_list_copy():", id(Week_list_copy1))

Running results :

Week_list: 250643256008
Week_list_copy(): 250643201352

It can be seen that : Use copy() The way to copy a function is the same as passing through [:] It's copied in a similar way , They all create a new list .


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