# sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: users.username

- **ID:** `python/flask-sqlalchemy-integrity-error`
- **Domain:** python
- **Category:** data_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

尝试插入重复的用户名，违反了数据库的唯一约束

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.9 | active | — | — |
| 3.10 | active | — | — |

## Workarounds

1. **在插入前检查用户名是否已存在** (90% success)
   ```
   if User.query.filter_by(username=username).first():
    return '用户名已存在', 400
   ```
2. **使用 try-except 捕获并返回友好错误** (95% success)
   ```
   try:
    db.session.add(user)
    db.session.commit()
except IntegrityError:
    db.session.rollback()
    return '用户名已存在', 400
   ```

## Dead Ends

- **捕获异常后直接忽略** — 用户不会收到错误提示，数据未保存 (90% fail)
- **删除唯一约束而不数据清洗** — 可能导致数据混乱 (70% fail)