# TypeError: assertRaises() 缺少 1 个必需的位置参数：'callable'

- **ID:** `python/unittest-assertraises-not-called`
- **领域:** python
- **类别:** type_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

unittest.TestCase.assertRaises 既可用作上下文管理器，也可传入可调用对象和参数。仅传入异常类而不传可调用对象（或未使用 `with`）会引发 TypeError。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 3.8 | active | — | — |
| 3.10 | active | — | — |
| 3.12 | active | — | — |

## 解决方案

1. **** (98% 成功率)
   ```
   with self.assertRaises(ValueError):
    int('not-a-number')
   ```
2. **** (95% 成功率)
   ```
   self.assertRaises(ValueError, int, 'not-a-number')
# or with keyword args:
self.assertRaises(ValueError, int, base=2, x='zz')
   ```

## 无效尝试

- **** — assertRaises expects an exception class or tuple of classes, not a string; passing a string causes a different TypeError about the expected type. (95% 失败率)
- **** — This actually works, but if the user forgets the second argument entirely (the common mistake), it still raises the same TypeError; the dead-end is the mental model of 'just add the exception class'. (80% 失败率)