# 运行时错误：会话不可用，因为未设置密钥。

- **ID:** `python/flask-session-modification-error`
- **领域:** python
- **类别:** runtime_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

Flask需要使用secret_key来使用会话，但未设置。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 3.x | active | — | — |

## 解决方案

1. **Set a secret key in the Flask app configuration.** (95% 成功率)
   ```
   app.secret_key = 'your-secret-key'
   ```
2. **Use environment variables for security.** (90% 成功率)
   ```
   import os
app.secret_key = os.environ.get('SECRET_KEY', 'fallback-key')
   ```

## 无效尝试

- **Setting secret_key to an empty string.** — An empty string is not a valid secret key; Flask will still raise an error. (80% 失败率)
- **Using secret_key after the first request without setting it before.** — Secret key must be set before the first request; setting it later has no effect. (70% 失败率)