十五天Python系统学习教程第四天

// Java异常处理  
try {  
    FileReader file = new FileReader("data.txt");  
} catch (FileNotFoundException e) {  
    System.out.println("文件未找到: " + e.getMessage());  
} finally {  
    file.close();  
}  

# Python异常处理  
try:  
    file = open("data.txt")  
except FileNotFoundError as e:  
    print(f"文件未找到: {e.strerror}")  
else:  
    print("文件打开成功")  
finally:  
    file.close()  

BaseException  
 ├── KeyboardInterrupt  
 ├── SystemExit  
 └── Exception  
      ├── IOError  
      ├── ValueError  
      └── ...  

try:  
    num = int(input("输入数字: "))  
except (ValueError, TypeError) as e:  
    print(f"输入错误: {e}")  

try:  
    connect_db()  
except ConnectionError as e:  
    raise ServerError("服务不可用") from e  

# 读取整个文件（对比Java Files.readAllBytes）  
with open("data.txt", "r", encoding="utf-8") as f:  
    content = f.read()  

# 逐行读取（内存友好）  
with open("large_file.txt") as f:  
    for line in f:  
        process(line)  

# 写入文件  
with open("output.txt", "w") as f:  
    f.write("Hello\nWorld")  

import csv  

# 读取CSV（对比Java OpenCSV）  
with open("sales.csv", "r") as f:  
    reader = csv.DictReader(f)  
    for row in reader:  
        print(row["product"], row["price"])  

# 写入CSV  
headers = ["name", "age"]  
data = [["Alice", 30], ["Bob", 25]]  
with open("output.csv", "w", newline="") as f:  
    writer = csv.writer(f)  
    writer.writerow(headers)  
    writer.writerows(data)  

class DatabaseConnection:  
    def __enter__(self):  
        self.conn = connect_db()  
        return self.conn  

    def __exit__(self, exc_type, exc_val, exc_tb):  
        self.conn.close()  
        if exc_type:  
            print(f"错误发生: {exc_val}")  

# 使用方式  
with DatabaseConnection() as conn:  
    conn.execute_query("SELECT ...")  

// Java自动资源管理  
try (Connection conn = DriverManager.getConnection(url)) {  
    Statement stmt = conn.createStatement();  
}  // 自动调用close()  

import csv  

def read_sales_data(file_path):  
    try:  
        with open(file_path, "r") as f:  
            reader = csv.DictReader(f)  
            if not reader.fieldnames:  
                raise ValueError("空文件或缺少表头")  
            return list(reader)  
    except FileNotFoundError:  
        print("销售文件不存在")  
        return []  

def calculate_total(sales):  
    total = 0.0  
    for row in sales:  
        try:  
            price = float(row["price"])  
            qty = int(row["quantity"])  
            total += price * qty  
        except (KeyError, ValueError) as e:  
            print(f"数据格式错误: {e}")  
    return total  

def save_report(total):  
    try:  
        with open("report.txt", "w") as f:  
            f.write(f"总销售额: {total:.2f}")  
    except IOError as e:  
        print(f"报告生成失败: {e.strerror}")  

import csv  

def main():  
    sales_data = read_sales_data("sales.csv")  
    if not sales_data:  
        return  

    total = calculate_total(sales_data)  
    save_report(total)  
    print("报告生成成功")  

if __name__ == "__main__":  
    main()  

def iter_sales(file_path):  
    with open(file_path) as f:  
        reader = csv.DictReader(f)  
        for row in reader:  
            yield row  

class SalesDataError(Exception):  
    def __init__(self, message, row_num):  
        super().__init__(message)  
        self.row_num = row_num  

raise SalesDataError("价格格式错误", 5)