result = db.fetchone() 为什么会报错

`result = db.fetchone()` 会报错的可能原因有很多，这里列举几个常见的情况：

1. 数据库连接没有成功建立，导致 `db` 为 `None` 或者是无效的数据库连接对象。
2. 数据库查询语句有误，导致 `fetchone()` 方法无法正确执行。需要检查 SQL 语句的语法和逻辑。
3. 数据库查询结果为空，也就是 `fetchone()` 方法返回 `None`，但是没有对返回值进行判断，导致后面的代码出现异常。
4. 数据库查询结果不止一行，`fetchone()` 方法只返回一行结果，此时需要使用 `fetchall()` 方法获取所有结果，或者使用 `fetchmany()` 方法设置返回结果的数量。

需要根据具体的错误信息和代码实现进行排查，才能确定具体的原因和解决方案。

相关问题

用python查询MySQL表联系人，查询结果不存在，result_label.config(text="联系人不存在，请重新输入！")，报错UnboundLocalError: cannot access local variable 'result_label' where it is not associated with a value

这个错误提示是因为在你的代码中，`result_label` 这个变量没有被赋值，但是你在对它进行操作，导致程序无法识别它的类型。你需要在代码中为 `result_label` 这个变量赋值，如下所示：

# 导入相关库
import mysql.connector
from tkinter import *

# 连接数据库
db = mysql.connector.connect(
  host="localhost",
  user="yourusername",
  password="yourpassword",
  database="yourdatabase"
)

# 创建游标
cursor = db.cursor()

# 查询联系人
sql = "SELECT * FROM contacts WHERE name = 'your_contact_name'"
cursor.execute(sql)
result = cursor.fetchone()

# 关闭游标和数据库
cursor.close()
db.close()

# 创建GUI窗口
root = Tk()

# 创建标签
result_label = Label(root, text="")

# 判断查询结果
if result:
    result_label.config(text="联系人存在！")
else:
    result_label.config(text="联系人不存在，请重新输入！")

# 显示标签
result_label.pack()

# 运行GUI窗口
root.mainloop()

在以上代码中，我们为 `result_label` 这个变量创建了一个 `Label` 标签，并在代码中为它赋值。这样就可以避免 `UnboundLocalError` 这个错误提示了。

网页存在注册信息保存不了，宠物如果名字为中文会报错的问题，请改完将全部代码发出

为了修复注册信息保存不了和宠物名字为中文时出现的报错问题，我们需要对代码进行一些调整。以下是修改后的完整代码：

### `app.py`

from flask import Flask, render_template, request, redirect, url_for, flash, session
import sqlite3
import bcrypt
import logging

app = Flask(__name__)
app.secret_key = 'your_secret_key'

# 配置日志
logging.basicConfig(level=logging.INFO, format='%(asctime)s - %(levelname)s - %(message)s')

# 数据库连接
def connect_db():
    conn = sqlite3.connect('pet_weight_management.db')
    conn.text_factory = str  # 解决中文字符问题
    return conn

# 初始化数据库
def init_db():
    conn = connect_db()
    cursor = conn.cursor()
    cursor.execute('''
        CREATE TABLE IF NOT EXISTS users (
            id INTEGER PRIMARY KEY AUTOINCREMENT,
            username TEXT UNIQUE NOT NULL,
            password TEXT NOT NULL
        )
    ''')
    cursor.execute('''
        CREATE TABLE IF NOT EXISTS pets (
            id INTEGER PRIMARY KEY AUTOINCREMENT,
            user_id INTEGER NOT NULL,
            pet_name TEXT NOT NULL,
            FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE CASCADE
        )
    ''')
    cursor.execute('''
        CREATE TABLE IF NOT EXISTS pet_weights (
            id INTEGER PRIMARY KEY AUTOINCREMENT,
            pet_id INTEGER NOT NULL,
            weight REAL NOT NULL,
            recorded_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
            FOREIGN KEY (pet_id) REFERENCES pets (id) ON DELETE CASCADE
        )
    ''')
    conn.commit()
    conn.close()

@app.route('/')
def index():
    return render_template('index.html')

@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        conn = connect_db()
        cursor = conn.cursor()
        hashed_password = bcrypt.hashpw(password.encode('utf-8'), bcrypt.gensalt())
        try:
            cursor.execute('INSERT INTO users (username, password) VALUES (?, ?)', (username, hashed_password))
            conn.commit()
            flash("注册成功！", 'success')
            logging.info("用户 %s 注册成功！", username)
            return redirect(url_for('login'))
        except sqlite3.IntegrityError:
            flash("用户名已存在！", 'warning')
            logging.warning("用户名 %s 已存在！", username)
        finally:
            conn.close()
    return render_template('register.html')

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        conn = connect_db()
        cursor = conn.cursor()
        cursor.execute('SELECT id, password FROM users WHERE username = ?', (username,))
        result = cursor.fetchone()
        if result and bcrypt.checkpw(password.encode('utf-8'), result[1]):
            session['user_id'] = result[0]
            flash("登录成功！", 'success')
            logging.info("用户 %s 登录成功！", username)
            return redirect(url_for('dashboard'))
        else:
            flash("用户名或密码错误！", 'danger')
            logging.warning("用户名或密码错误！")
        conn.close()
    return render_template('login.html')

@app.route('/logout')
def logout():
    session.pop('user_id', None)
    flash("已退出登录。", 'info')
    logging.info("已退出登录。")
    return redirect(url_for('index'))

@app.route('/dashboard')
def dashboard():
    if 'user_id' not in session:
        return redirect(url_for('login'))
    user_id = session['user_id']
    conn = connect_db()
    cursor = conn.cursor()
    cursor.execute('SELECT id, pet_name FROM pets WHERE user_id = ?', (user_id,))
    pets = cursor.fetchall()
    conn.close()
    return render_template('dashboard.html', pets=pets)

@app.route('/add_pet', methods=['GET', 'POST'])
def add_pet():
    if 'user_id' not in session:
        return redirect(url_for('login'))
    if request.method == 'POST':
        user_id = session['user_id']
        pet_name = request.form['pet_name']
        weight = float(request.form['weight'])
        conn = connect_db()
        cursor = conn.cursor()
        cursor.execute('INSERT INTO pets (user_id, pet_name) VALUES (?, ?)', (user_id, pet_name))
        pet_id = cursor.lastrowid
        cursor.execute('INSERT INTO pet_weights (pet_id, weight) VALUES (?, ?)', (pet_id, weight))
        conn.commit()
        flash("宠物添加成功，初始体重为 %.2f kg" % weight, 'success')
        logging.info("宠物 %s 添加成功，初始体重为 %.2f kg", pet_name, weight)
        conn.close()
        return redirect(url_for('dashboard'))
    return render_template('add_pet.html')

@app.route('/view_pet/<int:pet_id>')
def view_pet(pet_id):
    if 'user_id' not in session:
        return redirect(url_for('login'))
    conn = connect_db()
    cursor = conn.cursor()
    cursor.execute('SELECT pet_name FROM pets WHERE id = ?', (pet_id,))
    pet = cursor.fetchone()
    if not pet:
        flash("宠物不存在！", 'warning')
        logging.warning("宠物不存在！")
        return redirect(url_for('dashboard'))
    cursor.execute('SELECT weight, recorded_at FROM pet_weights WHERE pet_id = ? ORDER BY recorded_at', (pet_id,))
    weights = cursor.fetchall()
    conn.close()
    return render_template('view_pet.html', pet=pet, weights=weights)

@app.route('/update_pet_weight/<int:pet_id>', methods=['GET', 'POST'])
def update_pet_weight(pet_id):
    if 'user_id' not in session:
        return redirect(url_for('login'))
    if request.method == 'POST':
        weight = float(request.form['weight'])
        conn = connect_db()
        cursor = conn.cursor()
        cursor.execute('INSERT INTO pet_weights (pet_id, weight) VALUES (?, ?)', (pet_id, weight))
        conn.commit()
        flash("宠物体重更新成功！", 'success')
        logging.info("宠物体重更新成功！")
        conn.close()
        return redirect(url_for('view_pet', pet_id=pet_id))
    return render_template('update_pet_weight.html', pet_id=pet_id)

@app.route('/delete_account', methods=['GET', 'POST'])
def delete_account():
    if 'user_id' not in session:
        return redirect(url_for('login'))
    if request.method == 'POST':
        user_id = session['user_id']
        conn = connect_db()
        cursor = conn.cursor()
        try:
            cursor.execute('DELETE FROM pet_weights WHERE pet_id IN (SELECT id FROM pets WHERE user_id = ?)', (user_id,))
            cursor.execute('DELETE FROM pets WHERE user_id = ?', (user_id,))
            cursor.execute('DELETE FROM users WHERE id = ?', (user_id,))
            conn.commit()
            session.pop('user_id', None)
            flash("账号已注销。", 'success')
            logging.info("用户 %s 账号已注销。", user_id)
        except Exception as e:
            flash("注销账号失败: %s" % str(e), 'danger')
            logging.error("注销账号失败: %s", str(e))
        finally:
            conn.close()
        return redirect(url_for('index'))
    return render_template('delete_account.html')

if __name__ == '__main__':
    init_db()
    app.run(debug=True)

### HTML 模板

#### `templates/index.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>宠物体重管理系统</title>
</head>
<body>
    <h1>欢迎来到宠物体重管理系统</h1>
    <a href="{{ url_for('register') }}">注册</a>
    <a href="{{ url_for('login') }}">登录</a>
</body>
</html>

#### `templates/register.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>注册</title>
</head>
<body>
    <h1>注册</h1>
    {% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
    <ul>
        {% for category, message in messages %}
        <li class="{{ category }}">{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    <form method="post">
        <label for="username">用户名:</label>
        <input type="text" id="username" name="username" required><br>
        <label for="password">密码:</label>
        <input type="password" id="password" name="password" required><br>
        <button type="submit">注册</button>
    </form>
</body>
</html>

#### `templates/login.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>登录</title>
</head>
<body>
    <h1>登录</h1>
    {% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
    <ul>
        {% for category, message in messages %}
        <li class="{{ category }}">{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    <form method="post">
        <label for="username">用户名:</label>
        <input type="text" id="username" name="username" required><br>
        <label for="password">密码:</label>
        <input type="password" id="password" name="password" required><br>
        <button type="submit">登录</button>
    </form>
</body>
</html>

#### `templates/dashboard.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>仪表盘</title>
</head>
<body>
    <h1>欢迎，{{ session.username }}!</h1>
    <a href="{{ url_for('add_pet') }}">添加宠物</a>
    <a href="{{ url_for('logout') }}">退出登录</a>
    <a href="{{ url_for('delete_account') }}">注销账号</a>
    <h2>我的宠物</h2>
    {% if pets %}
    <ul>
        {% for pet in pets %}
        <li><a href="{{ url_for('view_pet', pet_id=pet[0]) }}">{{ pet[1] }}</a></li>
        {% endfor %}
    </ul>
    {% else %}
    <p>还没有添加宠物。</p>
    {% endif %}
</body>
</html>

#### `templates/add_pet.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>添加宠物</title>
</head>
<body>
    <h1>添加宠物</h1>
    {% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
    <ul>
        {% for category, message in messages %}
        <li class="{{ category }}">{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    <form method="post">
        <label for="pet_name">宠物名字:</label>
        <input type="text" id="pet_name" name="pet_name" required><br>
        <label for="weight">宠物体重 (kg):</label>
        <input type="number" step="0.01" id="weight" name="weight" required><br>
        <button type="submit">添加宠物</button>
    </form>
</body>
</html>

#### `templates/view_pet.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>{{ pet[0] }} 的体重记录</title>
</head>
<body>
    <h1>{{ pet[0] }} 的体重记录</h1>
    {% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
    <ul>
        {% for category, message in messages %}
        <li class="{{ category }}">{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    <a href="{{ url_for('update_pet_weight', pet_id=pet[0]) }}">更新体重</a>
    <table>
        <tr>
            <th>体重 (kg)</th>
            <th>记录时间</th>
        </tr>
        {% for weight in weights %}
        <tr>
            <td>{{ weight[0] }}</td>
            <td>{{ weight[1] }}</td>
        </tr>
        {% endfor %}
    </table>
</body>
</html>

#### `templates/update_pet_weight.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>更新宠物体重</title>
</head>
<body>
    <h1>更新宠物体重</h1>
    {% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
    <ul>
        {% for category, message in messages %}
        <li class="{{ category }}">{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    <form method="post">
        <label for="weight">新的体重 (kg):</label>
        <input type="number" step="0.01" id="weight" name="weight" required><br>
        <button type="submit">更新体重</button>
    </form>
</body>
</html>

#### `templates/delete_account.html`

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>注销账号</title>
</head>
<body>
    <h1>确认注销账号</h1>
    <p>确定要注销账号吗？此操作不可逆。</p>
    <form method="post">
        <button type="submit">确定</button>
        <a href="{{ url_for('dashboard') }}">取消</a>
    </form>
</body>
</html>

### 主要改动点
1. **解决中文字符问题**：在 `connect_db` 函数中设置 `conn.text_factory = str`，以确保 SQLite 支持中文字符。
2. **确保注册信息保存**：确保所有必要的字段都正确插入到数据库中，并且没有遗漏。

希望这些改动能解决问题。如果有其他问题，请随时告知。