import requests
from bs4 import BeautifulSoup

response = requests.get("https://stackoverflow.com/questions")
soup = BeautifulSoup(response.text, "html.parser")

questions = soup.select(".question-summary")
print(questions[0].attrs)
print(questions[0].get("id", 0))

for question in questions:
    print(question.select_one(".question-hyperlink").getText())
    print(question.select_one(".vote-count-post").getText())

from selenium import webdriver
import time

browser = webdriver.Chrome()
browser.set_window_size(1920, 1080)      # 设置窗口大小

browser.get("https://github.com")

signin_link = browser.find_element_by_link_text("Sign in")
signin_link.click()

username_box = browser.find_element_by_id("login_field")
username_box.send_keys("PurpleMStone")
password_box = browser.find_element_by_id("password")
password_box.send_keys("zq15071452268")
password_box.submit()

assert "PurpleMStone" in browser.page_source

time.sleep(0.5)

# profile_link = browser.find_element_by_class_name("user-profile-link")
# link_label = profile_link.get_attribute("innerHTML")
# assert "PurpleMStone" in link_label

browser.quit()

import PyPDF2

with open("first.pdf", "rb") as file:
    reader = PyPDF2.PdfFileReader(file)
    print(reader.numPages)
    page = reader.getPage(0)        # 获取第一页
    page.rotateClockwise(90)        # 页面旋转90度
    
    writer = PyPDF2.PdfFileWriter()
    writer.addPage(page)
    with open("rotated.pdf", "wb") as output:
        writer.write(output)

import PyPDF2

merger = PyPDF2.PdfFileMerger()
file_names = ["first.pdf", "second.pdf"]
for file_name in file_names:
    merger.append(file_name)
merger.write("combined.pdf")

import openpyxl

wb = openpyxl.load_workbook("transactions.xlsx")
print(wb.sheetnames)

sheet = wb["Sheet1"]

# wb.create_sheet(["Sheet2", 0])
# wb.remove_sheet(sheet)

cell = sheet["a1"]
# print(cell.row)
# print(cell.column)
# print(cell.coordinate)
# cell = sheet.cell(row=1, column=1)

print(sheet.max_row)
print(sheet.max_column)

for row in range(1, sheet.max_row + 1):
    for column in range(1, sheet.max_column + 1):
        cell = sheet.cell(row, column)
        # print(cell.value)

column = sheet["a"]
print(column)
cells = sheet["a:c"]    # column
print(cells)

print(sheet[1:3])       # row

sheet.append([1, 2, 3])  # add a row at the end of the sheet
# sheet.insert_rows insert_columns, delete_rows, delete_columns

wb.save("transaction2.xlsx")

import numpy as np

array = np.array([[1, 2, 3], [4, 5, 6]])
print(array)
print(type(np.array))
print(array.shape)

array = np.zeros((3, 4), dtype=int)
print(array)

array = np.ones((3, 4), dtype=int)
print(array)

array = np.full((3, 4), 5, dtype=int)
print(array)

array = np.random.random((3, 4))
print(array)
print(array[0, 0])

print(array > 0.2)

print(array[array > 0.2])

print(np.sum(array))
print(np.floor(array))
print(np.ceil(array))
print(np.round(array))

first = np.array([1, 2, 3])
second = np.array([1, 2, 3])
print(first + second)

import requests

response = requests.get("https://baidu.com")
print(response)

# setup.py
import setuptools
import setuptools
from pathlib import Path

setuptools.setup(
    name="stonepdf",
    version=1.0,
    long_description=Path("README.md").read_text(),
    packages=setuptools.find_packages(exclude=["tests", "data"]) # 忽略这两个
)

""" This module provides functions to convert a PDF to text. """


class Converter:
    """ A simple converter for converting PDFs to text. """

    def convert(self, path):
        """
        Convert the given PDF to text. 

        Parameters:
        path (str): The path to a PDF file.

        Returns:
        str: The content of the PDF file as text.
        """

        print("pdf2text")

from pathlib import Path

path = Path(r"ecommerce/__init__.py")  # create a path object
print(path.exists())    # file exists or not
print(path.is_file())   # is a file or not
print(path.is_dir())	# is a directory or not
print(path.name)
print(path.stem)        # return the file name without extension
print(path.suffix)      # return the extension
print(path.parent)      # parent folder

from pathlib import Path

path = Path(r"ecommerce/__init__.py")  	# create a path object

path = path.with_name("file.txt")	   	# 产生一个新path object，(文件尚未产生)
print(path)

print(path.absolute())

path = path.with_suffix(".txt")		 	# 产生一个新path object，(文件尚未产生)
print(path)

from pathlib import Path

path = Path("ecommerce")
path.exists()
path.mkdir()    # creat this directory
path.rmdir()    # remove this directory
path.rename("ecommerce2")   # rename

from pathlib import Path

path = Path("ecommerce")
print(path.iterdir())

paths = [p for p in path.iterdir()]
print(paths)

# filter: 只获取目录
paths = [p for p in path.iterdir() if p.is_dir()]
print(paths)

# 当前目录下符合条件的文件
py_files = [p for p in path.glob("*.py")]
print(py_files)

# recursively (包括子目录下符合条件的文件)
py_files = [p for p in path.rglob("*.py")]
print(py_files)

from pathlib import Path

path = Path("ecommerce/__init__.py")
path.exists()
path.rename("init.txt")
path.unlink() # delete the file

from pathlib import Path

print(path.stat())

from time import ctime

print(ctime(path.stat().st_ctime))	# 打印文件的新建时间

print(path.read_bytes())
print(path.read_text())
path.write_bytes("...")
path.write_text("...")

from pathlib import Path
import shutil

source = Path("ecommerce/__init__.py")
target = Path() / "__init__.py"

# target.write_text(source.read_text())		# 简单粗暴：读->写
shutil.copy(source, target)

from pathlib import Path
from zipfile import ZipFile

zip = ZipFile("files.zip", "w")					# 生成一个zip文件
for path in Path("ecommerce").rglob("*.*"):
    zip.write(path)								# 将这些文件存到zip文件中
zip.close()										# 关闭

from pathlib import Path
from zipfile import ZipFile

with ZipFile("files.zip", "w") as zip:
    for path in Path("ecommerce").rglob("*.*"):
        zip.write(path)

from pathlib import Path
from zipfile import ZipFile

with ZipFile("files.zip") as zip:
    # print(zip.namelist())								# zip中的文件列表
    info = zip.getinfo("ecommerce/__init__.py")			# 获取文件信息
    print(info.file_size)								# 文件大小
    print(info.compress_size)							# 压缩大小
    zip.extractall("extract")							# 解压到extract文件夹

import csv

with open("data.csv", "w", newline='') as file:
    writer = csv.writer(file)
    writer.writerow(["transaction_id", "product_id", "price"])
    writer.writerow([1000, 1, 5])
    writer.writerow([1001, 2, 15])

import csv

with open("data.csv") as file:
    reader = csv.reader(file)
    print(list(reader))

import csv

with open("data.csv") as file:
    reader = csv.reader(file)
    for row in reader:			# reader is iterable
        print(row)

import json
from pathlib import Path

movies = [
    {"id": 1, "title": "Terminator", "year": 1989},
    {"id": 2, "title": "Kindergarten Cop", "year": 1993},
]

data = json.dumps(movies)
Path("movies.json").write_text(data)

import json
from pathlib import Path

data = Path("movies.json").read_text()
movies = json.loads(data)
print(movies[0])

import sqlite3
import json
from pathlib import Path

movies = json.loads(Path("movies.json").read_text())

with sqlite3.connect("db.sqlite3") as conn:
    command = "INSERT INTO Movies VALUES(?, ?, ?)"
    for movie in movies:
        conn.execute(command, tuple(movie.values()))
    conn.commit()

import sqlite3

with sqlite3.connect("db.sqlite3") as conn:
    command = "SELECT * FROM Movies"
    cursor = conn.execute(command)
    for row in cursor:
        print(row)

import sqlite3

with sqlite3.connect("db.sqlite3") as conn:
    command = "SELECT * FROM Movies"
    cursor = conn.execute(command)
    movies = cursor.fetchall()
    print(movies)

import time


def send_emails():
    for i in range(1000000):
        pass


start = time.time()
send_emails()
end = time.time()
duration = end - start
print(duration)

from datetime import datetime
import time

dt1 = datetime(2018, 1, 1)
print(dt1)					# 2018-01-01 00:00:00
dt2 = datetime.now()
print(dt2)					# 2021-05-13 16:51:52.269074
print(dt2 > dt1)			# True

dt = datetime.strptime("2018/01/01", "%Y/%m/%d")    # 指定时间
print(dt)					# 2018-01-01 00:00:00

dt = datetime.fromtimestamp(time.time())
print(f"{dt.year}/{dt.month}")	# 2021/5
print(dt.strftime("%Y/%m"))     # 2021/05 (当地时间)

from datetime import datetime, timedelta

dt1 = datetime(2020, 1, 1) + timedelta(days=1, seconds=1000)
print(dt1)
dt2 = datetime.now()

duration = dt2 - dt1
print(duration)
print("days", duration.days)
print("seconds", duration.seconds)						# 不计天数
print("total_seconds", duration.total_seconds())		# 计天数

import random
import string

print(random.random())
print(random.randint(1, 10))
print(random.choice([1, 2, 3, 4]))				# 从给定的值中随机选一个
print(random.choices([1, 2, 3, 4], k=4))		# 从给定的值中随机选出序列
print("".join(random.choices("abcdefg", k=4)))

import random
import string

print(string.ascii_letters)		# 所有字母
print(string.digits)			# 所有数字
print("".join(random.choices(string.ascii_letters + string.digits, k=4)))

import random

numbers = [1, 2, 3, 4]
random.shuffle(numbers)
print(numbers)

import webbrowser

print("Deployment completed")
webbrowser.open("https://baidu.com")

from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.image import MIMEImage
from pathlib import Path
import smtplib

message = MIMEMultipart()
message["from"] = "name_of_sender"
message["to"] = "email_of_recipients@email.com"
message["subject"] = "This is a test"
message.attach(MIMEText("Body"))						# 正文
message.attach(MIMEImage(Path("ww.jpg").read_bytes()))	# 附件图片

with smtplib.SMTP(host="smtp.whu.edu.cn", port=25) as smtp:
    smtp.ehlo()      # say hello to smtp server
    smtp.starttls()  # TLS
    smtp.login("email_of_sender@email.com", "password")
    smtp.send_message(message)
    print("Sent...")

from string import Template

template = Template(Path("template.html").read_text())		# 读入模板

# 相应语句改为
body = template.substitute({"name": "John"}) 	# 或者name="John"
message.attach(MIMEText(body, "html"))			# "html"

import sys

print(sys.argv)

import sys

if len(sys.argv) == 1:
    print("USAGE: python3 app.py <password>")
else:
    password = sys.argv[1]
    print("password", password)

import subprocess

completed = subprocess.run(["dir", "."], 
                           shell=True,
                           capture_output=True, 
                           text=True)
print(type(completed))

print("args", completed.args)
print("returncode", completed.returncode)   # 0: success
print("stderr", completed.stderr)       # standard error
print("stdout", completed.stdout)       # standard output

# other.py
print("Here is a complicated script.")

# app.py
import subprocess

completed = subprocess.run(["python", "other.py"], 
                           shell=True,
                           capture_output=True, 
                           text=True)
print("args", completed.args)
print("returncode", completed.returncode)   # 0: success
print("stderr", completed.stderr)       # standard error
print("stdout", completed.stdout)       # standard output

import subprocess

completed = subprocess.run(["False"], 
                           shell=True,
                           capture_output=True, 
                           text=True)
print("args", completed.args)
print("returncode", completed.returncode)   # 0: success
print("stderr", completed.stderr)       # standard error
print("stdout", completed.stdout)       # standard output

if completed.returncode != 0:		# 可以这样判断处理
    print(completed.stderr)

import subprocess

try:
    completed = subprocess.run(["False"],
                               shell=True,
                               capture_output=True,
                               text=True,
                               check=True)		# 自动检查
    print("args", completed.args)
    print("returncode", completed.returncode)   # 0: success
    print("stderr", completed.stderr)       # standard error
    print("stdout", completed.stdout)       # standard output

except subprocess.CalledProcessError as ex:
    print(ex)

# sales.py
def calc_tax():
    pass


def calc_shipping():
    pass

# app.py
from sales import calc_shipping, calc_tax

calc_shipping()
calc_tax()

import sales

sales.calc_shipping()
sales.calc_tax()

import sys

print(sys.path)

['e:\\Stone\\Python\\HelloWorld', 'D:\\Python39\\python39.zip', 'D:\\Python39\\DLLs', 'D:\\Python39\\lib', 'D:\\Python39', 'C:\\Users\\11405\\AppData\\Roaming\\Python\\Python39\\site-packages', 'D:\\Python39\\lib\\site-packages']

# app.py
from ecommerce import sales

sales.calc_tax()
sales.calc_shipping()

# app.py
from ecommerce.shopping import sales

# sales.py
from ecommerce.customer import contact
from ..customer import contact		# ..表示父目录

contact.contact_customer()

# app.py
from ecommerce.shopping import sales

print(dir(sales))

['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'calc_shipping', 'calc_tax', 'contact']

# app.py
from ecommerce.shopping import sales

print(sales.__name__)
print(sales.__package__)
print(sales.__file__)

ecommerce.shopping.sales
ecommerce.shopping
e:\Stone\Python\HelloWorld\ecommerce\shopping\sales.py

# ecommerce/shopping/sales.py
print("Sales initialized", __name__)


def calc_tax():
    pass


def calc_shipping():
    pass

# ecommerce/__init__.py
print("Ecommerce initialized")

# app.py
from ecommerce.shopping import sales

Ecommerce initialized
Sales initialized ecommerce.shopping.sales

Sales initialized __main__

if __name__ == "__main__":
    # bla bla bla

class Point:
    def draw(self):
        print("draw")


point = Point()
print(type(point))

print(isinstance(point, Point)) 	# 判断point是不是Point的实例

<class '__main__.Point'>
True

class Point:
    def __init__(self, x, y):		# Constructor
        self.x = x
        self.y = y

    def draw(self):
        print(f"Point ({self.x}, {self.y})")


point = Point(1, 2)
print(point.x)
point.draw()

1
Point (1, 2)

class Point:
    default_color = "red"			# class attribute

    def __init__(self, x, y):
        self.x = x					# object/instance attribute
        self.y = y

    def draw(self):
        print(f"Point ({self.x}, {self.y})")


point = Point(1, 2)
point.draw()						# x和y是object attribute，每个object不同

another = Point(3, 4)
another.draw()

Point.default_color = "yellow"		# 直接用类调用更改。类中所有实例共享
print(point.default_color)
print(Point.default_color)
print(another.default_color)

Point (1, 2)
Point (3, 4)
yellow
yellow
yellow

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    @classmethod
    def zero(cls):      	# cls指向这个class本身
        return cls(0, 0)   	# Point(0, 0)

    def draw(self):
        print(f"Point ({self.x}, {self.y})")


point = Point.zero()		# 工厂方法:将复杂的对象构造(比如多种初始值)交给类方法去完成
point.draw()				# 输出 Point (0, 0)

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def draw(self):
        print(f"Point ({self.x}, {self.y})")


point = Point(1, 2)
print(point)
print(str(point))

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __str__(self):
        return f"({self.x}, {self.y})"

    def draw(self):
        print(f"Point ({self.x}, {self.y})")


point = Point(1, 2)
print(point)
print(str(point))

(1, 2)
(1, 2)

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y


point = Point(1, 2)
other = Point(1, 2)
print(point == other)

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

    def __gt__(self, other):
        return self.x > other.x and self.y > other.y


point = Point(10, 20)
other = Point(1, 2)
print(point == other)
print(point > other)

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        return Point(self.x + other.x, self.y + other.y)


point = Point(10, 20)
other = Point(1, 2)
combined = point + other
print(f"Point ({combined.x}, {combined.y})")

Point (11, 22)

class TagCloud:
    def __init__(self):
        self.tags = {}

    def add(self, tag):
        self.tags[tag.lower()] = self.tags.get(tag.lower(), 0) + 1

    def __getitem__(self, tag):
        return self.tags.get(tag.lower(), 0)

    def __setitem__(self, tag, count):
        self.tags[tag.lower()] = count

    def __len__(self):
        return len(self.tags)

    def __iter__(self):     # make it iterable
        return iter(self.tags)


cloud = TagCloud()
cloud.add("Python")
cloud.add("python")
cloud.add("python")
print(cloud.tags)

print(cloud["python"])  # __getitem__实现
cloud["java"] = 10      # __setitem__实现
print(cloud.tags)

print(len(cloud))       # __len__实现

{'python': 3}
3
{'python': 3, 'java': 10}
2

class TagCloud:
    def __init__(self):
        self.__tags = {}


cloud = TagCloud()
print(cloud.__dict__)
print(cloud._TagCloud__tags)
print(cloud.__tags)

{'_TagCloud__tags': {}}
{}
Traceback (most recent call last):
  File "e:\HelloWorld\app.py", line 24, in <module>
    print(cloud.__tags)
AttributeError: 'TagCloud' object has no attribute '__tags'

class Product:
    def __init__(self, price):
        self.set_price(price)

    def get_price(self):
        return self.__price

    def set_price(self, value):
        if value < 0:
            raise ValueError("Price cannot be negative.")
        self.__price = value


product = Product(-50)

class Product:
    def __init__(self, price):
        self.price = price

    @property
    def price(self):
        return self.__price

    @price.setter
    def price(self, value):
        if value < 0:
            raise ValueError("Price cannot be negative.")
        self.__price = value

    # price = property(get_price, set_price)


product = Product(-10)
print(product.price)

class Animal:
    def __init__(self):
        self.age = 1

    def eat(self):
        print("eat")


# Animal: Parent, Base
# Mammal: Child, Sub
class Mammal(Animal):
    def walk(self):
        print("walk")


class Fish(Animal):
    def swim(self):
        print("swim")


m = Mammal()
m.eat()
print(m.age)

eat
1

class Animal:
    def __init__(self):
        self.age = 1

    def eat(self):
        print("eat")


# Animal: Parent, Base
# Mammal: Child, Sub
class Mammal(Animal):
    def walk(self):
        print("walk")


m = Mammal()
print(isinstance(m, Animal))		# True
print(isinstance(m, Mammal))		# True
print(isinstance(m, object))		# True	
print(issubclass(Mammal, Animal))	# True
print(issubclass(Mammal, object))	# True

class Animal:
    def __init__(self):
        print("Animal Constructor")
        self.age = 1

    def eat(self):
        print("eat")


# Animal: Parent, Base
# Mammal: Child, Sub
class Mammal(Animal):
    def __init__(self):
        super().__init__() 		# 调用父类的__init__(),否则会被子类的覆盖掉而不执行
        print("Mammal Constructor")
        self.weight = 2

    def walk(self):
        print("walk")


m = Mammal()
print(m.age)
print(m.weight)

Animal Constructor
Mammal Constructor
1
2

class Employee:
    def greet(self):
        print("Employee Greet")


class Person:
    def greet(self):
        print("Person Greet")


class Manager(Employee, Person):
    pass


manager = Manager()
manager.greet()

class Flyer:
    def fly(self):
        pass


class Swimmer:
    def swim(self):
        pass


class FlyingFish(Flyer, Swimmer):
    pass

class InvalidOperationError(Exception):
    pass


class Stream:
    def __init__(self):
        self.opened = False

    def open(self):
        if self.opened:
            raise InvalidOperationError("Stream is already opened.")
        self.opened = True

    def close(self):
        if not self.opened:
            raise InvalidOperationError("Stream is already closed.")
        self.opened = False


class FileStream(Stream):
    def read(self):
        print("Reading data from a file.")


class NetworkStream(Stream):
    def read(self):
        print("Reading data from a network.")

from abc import ABC, abstractmethod


class InvalidOperationError(Exception):
    pass


class Stream(ABC):					# 继承自ABC(抽象类简写)
    def __init__(self):
        self.opened = False

    def open(self):
        if self.opened:
            raise InvalidOperationError("Stream is already opened.")
        self.opened = True

    def close(self):
        if not self.opened:
            raise InvalidOperationError("Stream is already closed.")
        self.opened = False

    @abstractmethod
    def read(self):			# 抽象方法
        pass


class FileStream(Stream):
    def read(self):
        print("Reading data from a file.")


class NetworkStream(Stream):
    def read(self):
        print("Reading data from a network.")


class MemoryStream(Stream):
    def read(self):
        print("Reading data from the memory.")


stream = MemoryStream()
stream.read()

from abc import ABC, abstractmethod


class UIControl(ABC):
    @abstractmethod
    def draw(self):
        pass


class TextBox(UIControl):
    def draw(self):
        print("TextBox")


class DropDownList(UIControl):
    def draw(self):
        print("DropDownList")


def draw(controls):					# 在run-time才知道control具体是什么
    for control in controls:
        control.draw()


ddl = DropDownList()
textbox = TextBox()
draw([ddl, textbox])				# 多种对象实现一种方法

DropDownList
TextBox

from abc import ABC, abstractmethod


class TextBox:
    def draw(self):
        print("TextBox")


class DropDownList:
    def draw(self):
        print("DropDownList")


def draw(controls):
    for control in controls:
        control.draw()


ddl = DropDownList()
textbox = TextBox()
draw([ddl, textbox])

class Text(str):
    def duplicate(self):
        return self + self


text = Text("Python")
print(text.duplicate())	# 本类方法
print(text.lower())     # 父类方法

class TrackableList(list):
    def append(self, object):
        print("Append called")
        super().append(object)		# 调用父类方法


list = TrackableList()
list.append("1")

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __eq__(self, other) -> bool:
        return self.x == other.x and self.y == other.y


p1 = Point(1, 2)
p2 = Point(1, 2)
print(p1 == p2)

from collections import namedtuple

Point = namedtuple("Point", ["x", "y"])
p1 = Point(x=1, y=2)        # must be keyword arguments
p2 = Point(x=1, y=2)

print(p1 == p2)				# True
print(p1.x)					# 像类一样调用：1

p1.x = 10					# AttributeError: can't set attribute

p1 = Point(x=10, y=2)

try:
    age = int(input("Age: "))
except ValueError as ex:
    print("You didn't enter a valid age.")
    print(ex)			# 输出错误信息
    print(type(ex))		# 显示错误类型
else:					# 没有exception就执行这一块
    print("No exceptions were thrown.")

print("Execution continues.")

>python app.py
Age: 10
No exceptions were thrown.
Execution continues.

>python app.py
Age: a
You didn't enter a valid age.
invalid literal for int() with base 10: 'a'   
<class 'ValueError'>
Execution continues.

try:
    age = int(input("Age: "))
    xfactor = 10 / age
except (ValueError, ZeroDivisionError):			# 多种exception之一即执行
    print("You didn't enter a valid age.")
else:
    print("No exceptions were thrown.")

> python app.py
Age: 0
You didn't enter a valid age.

try:
    file = open("app.py")
    age = int(input("Age: "))
    xfactor = 10 / age
except (ValueError, ZeroDivisionError):
    print("You didn't enter a valid age.")
else:
    print("No exceptions were thrown.")
finally:
    file.close()

with open("app.py") as file:
       print("File opened.")

def calculate_xfactor(age):
    if age <= 0:
        raise ValueError("Age cannot be 0 or less.")
    return 10 / age


try:
    calculate_xfactor(-1)
except ValueError as error:
    print(error)

from timeit import timeit

code1 = """
def calculate_xfactor(age):
    if age <= 0:
        raise ValueError("Age cannot be 0 or less.")
    return 10 / age


try:
    calculate_xfactor(-1)
except ValueError as error:
    pass
"""

code2 = """
def calculate_xfactor(age):
    if age <= 0:
        return None
    return 10 / age


xfactor = calculate_xfactor(-1)
if xfactor == None:
    pass
"""

print("first code=", timeit(code1, number=10000))
print("second code=", timeit(code2, number=10000))

first code= 0.0040202000000000016
second code= 0.0015166999999999958

letters = ["a", "b", "c"]
matrix = [[0, 1], [2, 3]]

zeros = [0] * 5             # [0,0,0,0,0]
combined = zeros + letters  # [0, 0, 0, 0, 0, 'a', 'b', 'c']
numbers = list(range(20))
chars = list("Hello World")	# ['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd']
print(len(chars))           # 11

letters = ["a", "b", "c", "d"]
print(letters[0])
print(letters[-1])
letters[0] = "A"
print(letters)
print(letters[:3])
print(letters[::2])

a
d
['A', 'b', 'c', 'd']
['A', 'b', 'c']
['A', 'c']

numbers = list(range(10))
print(numbers)
print(numbers[::2])
print(numbers[::-1])		# 倒序输出

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 2, 4, 6, 8]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

numbers = [1, 2, 3]
first, second, third = numbers		# first=1, second=2, third=3
first, second, _ = numbers

numbers2 = [1, 2, 3, 4, 4, 4, 4, 4, 4]
first, second, *other = numbers2
print(first)		# 1
print(other)		# [3, 4, 4, 4, 4, 4, 4]