Python 快速入门：JavaScript 开发者指南

# Python 使用 = 赋值，没有 let/const/var
name = "小明"          # 字符串
age = 25               # 整数
price = 19.99          # 浮点数
is_active = True       # 布尔值 (首字母大写)

# 常量约定：全大写
MAX_SIZE = 100

# 多变量赋值
x, y, z = 1, 2, 3

// JS
const name = "小明";
let age = 25;
var is_active = true;

# 字符串
s = "Hello"
s = 'Hello'
s = """多行
字符串"""        # 类似 JS 的 template literal

# 数字
int_num = 42       # 整数
float_num = 3.14   # 浮点数
complex_num = 3+4j # 复数

# 布尔值
is_python_cool = True  # 注意首字母大写
is_js_better = False

# None (类似 JS 的 null)
result = None

# 字符串转数字
int("42")        # 42
float("3.14")    # 3.14

# 数字转字符串
str(42)          # "42"

# 字符串转布尔值
bool("")         # False (空字符串为 False)
bool("hello")    # True (非空字符串为 True)
bool(0)          # False
bool(1)          # True

# 创建列表
fruits = ["apple", "banana", "orange"]

# 访问元素 (0 索引)
fruits[0]        # "apple"
fruits[-1]       # "orange" (倒数第一个)

# 切片操作 (类似 JS 的 slice)
fruits[1:3]      # ["banana", "orange"]
fruits[:2]       # ["apple", "banana"]
fruits[::2]      # ["apple", "orange"] (步长2)

# 修改列表
fruits.append("grape")      # 末尾添加
fruits.insert(1, "mango")   # 插入
fruits.remove("apple")      # 删除
fruits.pop()                # 弹出末尾

# 列表推导式 (Python 特色)
squares = [x**2 for x in range(10)]  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# 元组创建
point = (10, 20)
colors = ("red", "green", "blue")

# 解包 (类似 JS 解构)
x, y = point  # x=10, y=20

# 元组不可修改，但可以重新赋值
# point[0] = 15  # 报错！

# 创建字典
user = {
    "name": "小明",
    "age": 25,
    "is_active": True
}

# 访问值
user["name"]           # "小明"
user.get("email", "default@example.com")  # 类似 || 操作符

# 修改字典
user["age"] = 26           # 修改
user["email"] = "x@com"    # 添加
del user["age"]            # 删除
user.pop("is_active")      # 弹出删除

# 字典推导式
names = ["alice", "bob", "charlie"]
name_lengths = {name: len(name) for name in names}
# {"alice": 5, "bob": 3, "charlie": 8}

# 创建集合
fruits = {"apple", "banana", "orange"}

# 操作
fruits.add("grape")        # 添加
fruits.remove("apple")     # 删除
fruits.pop()               # 随机弹出一个

# 集合运算
set1 = {1, 2, 3}
set2 = {2, 3, 4}

set1 | set2    # 并集: {1, 2, 3, 4}
set1 & set2    # 交集: {2, 3}
set1 - set2    # 差集: {1}

age = 18

if age >= 18:
    print("成年人")
elif age >= 13:
    print("青少年")
else:
    print("儿童")

# 三元表达式 (类似 JS 的 ?:)
status = "成年" if age >= 18 else "未成年"

# for 循环 (类似 JS for...of)
fruits = ["apple", "banana", "orange"]

for fruit in fruits:
    print(fruit)

# range (类似 JS 的 for 循环)
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

for i in range(1, 6):
    print(i)  # 1, 2, 3, 4, 5

# 遍历字典
user = {"name": "小明", "age": 25}
for key, value in user.items():
    print(f"{key}: {value}")

# 列表推导式代替 map/filter
numbers = [1, 2, 3, 4, 5]

# map
doubled = [x * 2 for x in numbers]  # [2, 4, 6, 8, 10]

# filter
evens = [x for x in numbers if x % 2 == 0]  # [2, 4]

# 类似 JS switch，但更强大
status = "pending"

match status:
    case "pending":
        print("待处理")
    case "success":
        print("成功")
    case "error":
        print("错误")
    case _:
        print("未知状态")  # default

# 定义函数 (类似 JS 函数声明)
def greet(name):
    return f"Hello, {name}!"

# 调用
result = greet("小明")  # "Hello, 小明!"

# 默认参数 (类似 JS)
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("小明")           # "Hello, 小明!"
greet("小明", "Hi")    # "Hi, 小明!"

# 返回多个值 (实际返回元组)
def get_user():
    return "小明", 25, "北京"

name, age, city = get_user()  # 解包

# *args 可变位置参数 (类似 JS 的 rest 参数)
def sum(*args):
    total = 0
    for num in args:
        total += num
    return total

sum(1, 2, 3, 4)  # 10

# **kwargs 可变关键字参数
def configure(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

configure(host="localhost", port=8080)
# host: localhost
# port: 8080

# lambda (类似 JS 箭头函数)
add = lambda x, y: x + y
add(1, 2)  # 3

# 结合内置函数
numbers = [1, 2, 3, 4, 5]
sorted(numbers, key=lambda x: -x)  # [5, 4, 3, 2, 1]

class User:
    # 类属性
    species = "Human"
    
    # 构造方法 (类似 JS constructor)
    def __init__(self, name, age):
        # 实例属性
        self.name = name
        self.age = age
    
    # 实例方法 (self 类似 JS 的 this)
    def greet(self):
        return f"Hello, I'm {self.name}"
    
    # 类方法
    @classmethod
    def create_anonymous(cls):
        return cls("Anonymous", 0)
    
    # 静态方法
    @staticmethod
    def is_adult(age):
        return age >= 18

# 创建实例
user = User("小明", 25)
user.greet()  # "Hello, I'm 小明"
User.is_adult(18)  # True

class Animal:
    def __init__(self, name):
        self.name = name
    
    def speak(self):
        pass

class Dog(Animal):
    def speak(self):
        return f"{self.name} says woof!"

dog = Dog("旺财")
dog.speak()  # "旺财 says woof!"

# @dataclass 自动生成 __init__, __repr__ 等
from dataclasses import dataclass

@dataclass
class User:
    name: str
    age: int
    email: str = "default@example.com"  # 默认值

user = User("小明", 25)
print(user)  # User(name='小明', age=25, email='default@example.com')

import asyncio

# 定义异步函数 (类似 JS async function)
async def fetch_data():
    print("开始获取数据...")
    await asyncio.sleep(1)  # 模拟异步操作
    print("数据获取完成!")
    return {"data": "hello"}

# 运行异步函数
async def main():
    result = await fetch_data()
    print(result)

# 方式1: asyncio.run (推荐)
asyncio.run(main())

# 方式2: 在已有事件循环中
# await fetch_data()

import asyncio

async def task1():
    await asyncio.sleep(1)
    return "任务1完成"

async def task2():
    await asyncio.sleep(2)
    return "任务2完成"

async def main():
    # 并发执行 (类似 Promise.all)
    results = await asyncio.gather(task1(), task2())
    print(results)  # ["任务1完成", "任务2完成"]

asyncio.run(main())

async def async_generator():
    for i in range(3):
        await asyncio.sleep(0.5)
        yield i

async def main():
    async for value in async_generator():
        print(value)

asyncio.run(main())

# 导入整个模块
import math
math.sqrt(16)  # 4.0

# 导入特定函数
from math import sqrt, pi
sqrt(16)  # 4.0

# 重命名
import numpy as np
np.array([1, 2, 3])

# 导入所有 (不推荐)
# from math import *

# mymodule.py
def greet(name):
    return f"Hello, {name}!"

# 同一目录下使用
import mymodule
mymodule.greet("小明")

# 安装包
pip install requests

# requirements.txt
# requests==2.28.0
# numpy>=1.21.0

# 安装依赖
pip install -r requirements.txt

# try-except (类似 try-catch)
try:
    result = 10 / 0
except ZeroDivisionError:
    print("不能除以0!")
except Exception as e:
    print(f"其他错误: {e}")
else:
    print("没有错误")
finally:
    print("总是执行")

# 抛出异常 (类似 throw)
raise ValueError("无效的值")

# 类型标注 (类似 TypeScript)
def greet(name: str) -> str:
    return f"Hello, {name}!"

# 复杂类型
from typing import List, Dict, Optional, Union

def process_items(items: List[int]) -> Dict[str, int]:
    return {
        "sum": sum(items),
        "count": len(items)
    }

# Optional 类似 union with null
def get_name(user_id: int) -> Optional[str]:
    if user_id > 0:
        return "小明"
    return None

# Union 类似 union 类型
def parse(value: Union[str, int]) -> str:
    return str(value)