# ModuleNotFoundError: No module named 'flask'

- **ID:** `python/flask-import-error-missing-flask`
- **Domain:** python
- **Category:** module_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Flask 库未安装或未在当前 Python 环境中激活

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.9 | active | — | — |
| 3.10 | active | — | — |
| 3.11 | active | — | — |

## Workarounds

1. **在项目虚拟环境中安装 Flask** (95% success)
   ```
   pip install flask
   ```
2. **创建并激活虚拟环境后安装** (98% success)
   ```
   python -m venv venv && source venv/bin/activate && pip install flask
   ```

## Dead Ends

- **尝试在未激活的虚拟环境中运行 pip install flask** — pip 安装到了系统级环境，而非当前项目虚拟环境 (70% fail)
- **手动复制 Flask 文件夹到项目目录** — 缺少依赖库，且版本不匹配 (85% fail)