# 抛出异常

## raise

使用 raise 语句抛出一个指定的异常。

```python
raise [Exception [, args [, traceback]]]
```

raise 唯一的一个参数指定了要被抛出的异常。它必须是一个异常的实例或者是异常的类（也就是 Exception 的子类）。

实例:

```python
# 如果 x 大于 5 就触发异常
x = 10
if x > 5:
    raise Exception('x 不能大于 5。x 的值为: {}'.format(x))
```

如果你只想知道这是否抛出了一个异常，并不想去处理它，那么一个简单的 raise 语句就可以再次把它抛出。

```python
>>> try:
        raise NameError('HiThere')
    except NameError:
        print('An exception flew by!')
        raise
   
An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
NameError: HiThere
```

## assert

assert（断言）用于判断一个表达式，在表达式条件为 false 的时候触发异常。

语法格式如下：

```python
assert expression
```

等价于：

```python
if not expression:
    raise AssertionError
```

assert 后面也可以紧跟参数:

```python
assert expression [, arguments]
```

等价于：

```python
if not expression:
    raise AssertionError(arguments)
```

实例1：

```python
>>> assert True     # 条件为 true 正常执行
>>> assert False    # 条件为 false 触发异常
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> assert 1==1    # 条件为 true 正常执行
>>> assert 1==2    # 条件为 false 触发异常
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

>>> assert 1==2, '1 不等于 2'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: 1 不等于 2
```

实例2：

```python
import sys
assert ('linux' in sys.platform), "该代码只能在 Linux 下执行"

# 判断当前系统是否为 Linux，如果不满足条件则直接触发异常，不必执行接下来的代码
# 接下来要执行的代码
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://lujw666.gitbook.io/learn/python/00-basic/00-cuo-wu-yu-yi-chang/03-pao-chu-yi-chang.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.