Article directory
list comprehension
The so-called list comprehension refers to the lightweight loop to create a list
- basic way
-
Use if in the loop
-
2 for loops
-
3 for loops
practise
Please write a piece of Python code to group the elements in a list, such as [1,2,3,…100] becomes [[1,2,3],[4,5,6]…]
Reference answer:
a = [x for x in range(1,101)]
b = [a[x:x+3] for x in range(0,len(a),3)]
copy of list
Look at the following code and say what is printed.
a = 12
b = a
b = 13
print(b)
print(a)
The output is:
13
12
nums1 = [1, 5, 8, 9, 10, 12]
nums2 = nums1
nums2[0] = 100
print(nums2)
print(nums1)
The output is:
[100, 5, 8, 9, 10, 12]
[100, 5, 8, 9, 10, 12]
think:
Why does the data in nums1 change when the data in nums2 is modified?
Assignment operations in Python are all transfers of references (that is, memory addresses). For mutable types, modifying the value of the original data will change the value of the assigned object.
How can nums1 and nums2 become two mutually independent and unaffected lists?
Use the list's copy method, or the copy module to assign a list.
The copy method of the list
Using the copy method of the list, you can directly copy the original list into a new list. This copy method is shallow copy.
nums1 = [1, 5, 8, 9, 10, 12]
nums2 = nums1.copy() # 调用列表的copy方法，可以复制出一个新的列表
nums2[0] = 100
# 修改新列表里的数据，不会影响到原有列表里的数据
print(nums2)
print(nums1)
Use of the copy module
In addition to using the copy method of the list, Python also provides the copy module to copy an object. The copy module provides two methods of shallow copy and deep copy. They are used in the same way, but the effect of execution is different.
shallow copy
Shallow copy is a top-level copy of an object. The popular understanding is: the reference is copied, but the content is not copied.
import copy
words1 = ['hello', 'good', ['yes', 'ok'], 'bad']
# 浅拷贝只会拷贝最外层的对象，里面的数据不会拷贝，而是直接指向
words2 = copy.copy(words1)
words2[0] = '你好'
words2[2][0] = 'no'
print(words1) # ['hello', 'good', ['no', 'ok'], 'bad']
# wrods2 里的 yes 被修改成了 no
print(words2) # ['你好', 'good', ['no', 'ok'], 'bad']
deep copy
A deep copy is a recursive copy of all levels of an object.
import copy
words1 = ['hello', 'good', ['yes', 'ok'], 'bad']
# 深拷贝会将对象里的所有数据都进行拷贝
words2 = copy.deepcopy(words1)
words2[0] = '你好'
words2[2][0] = 'no'
print(words1) # ['hello', 'good', ['yes', 'ok'], 'bad']
print(words2) # ['你好', 'good', ['no', 'ok'], 'bad']
slice
Lists, like strings, also support slicing, which is actually a shallow copy.
words1 = ['hello', 'good', ['yes', 'ok'], 'bad']
words2 = words1[:]
words2[0] = '你好'
words2[2][0] = 'no'
print(words1) # ['hello', 'good', ['no', 'ok'], 'bad']
print(words2) # ['你好', 'good', ['no', 'ok'], 'bad']
Advanced case
[Python] Python realizes the word guessing game-challenge your intelligence and luck!
[python] Python tkinter library implements GUI program for weight unit converter
[python] Use Selenium to get (2023 Blog Star) entries
Use Tencent Cloud Cloud studio to realize scheduling Baidu AI to realize text recognition
[Xiaobai must see] Python word cloud generator detailed analysis and code implementation
[Xiaobai must see] Python crawls an example of NBA player data
[Must-see for Xiaobai] Sample code for crawling and saving Himalayan audio using Python
[Xiaobai must see] Python crawler data processing and visualization
[Must-see for Xiaobai] Python crawler program to easily obtain hero skin pictures of King of Glory
[Must-see for Xiaobai] Use Python to generate a personalized list Word document
Geospatial data processing and visualization using geopandas and shapely (.shp)
Use selenium to crawl Maoyan movie list data
Detailed explanation of the principle and implementation of image enhancement algorithm Retinex
Getting Started Guide to Crawlers (8): Write weather data crawler programs for visual analysis
Getting started with crawlers (4): The best way to crawl dynamic web pages using Selenium and API
Getting Started Guide to Crawlers (3): Python network requests and common anti-crawler strategies
Getting started with crawlers (2): How to use regular expressions for data extraction and processing
Getting started with reptiles (1): Learn the basics and skills of reptiles
Python object-oriented programming basics and sample code
MySQL database operation guide: learn how to use Python to add, delete, modify and query operations
Python file operation guide: encoding, reading, writing and exception handling
Selenium Automation Toolset - Complete Guide and Tutorials
Python web crawler basics advanced to actual combat tutorial
Pandas data processing and analysis tutorial: from basics to actual combat
Detailed explanation of commonly used data types and related operations in Python
[Latest in 2023] Detailed Explanation of Six Major Schemes to Improve Index of Classification Model
Use 4 regression methods to draw prediction result charts: vector regression, random forest regression, linear regression, K-nearest neighbor regression
** [Learn python from zero] 18. Detailed explanation of basic operations of Python lists (1) **