187 lines
3.2 KiB
Markdown
187 lines
3.2 KiB
Markdown
# 09 - 异常处理
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Viper 支持 Python 风格的 `try/except/finally` 异常处理，编译为基于 jmp_buf 和错误码的 C 级别异常处理机制。
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## 异常处理机制
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Viper 的异常处理不使用 C++ 风格的零开销异常，而是通过函数参数传递错误码和错误消息实现。编译器自动为可能抛出异常的函数添加 `__eh_msg_out__` 和 `__eh_code_out__` 参数。
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## try / except
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```python
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try:
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result = risky_operation()
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except ValueError:
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serial.puts("ValueError occurred\n")
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except OSError:
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serial.puts("OSError occurred\n")
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except Exception:
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serial.puts("Unknown error\n")
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```
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### 内置异常类型
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Viper 预定义了以下异常类型及其错误码：
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| 异常类型 | 错误码 |
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|---------|--------|
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| `ValueError` | 1 |
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| `TypeError` | 2 |
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| `RuntimeError` | 3 |
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| `ZeroDivisionError` | 4 |
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| `IndexError` | 5 |
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| `KeyError` | 6 |
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| `IOError` | 7 |
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| `OSError` | 8 |
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| `AssertionError` | 9 |
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| `Exception` | 99 |
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### 自定义异常
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```python
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class DiskFullError(Exception):
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pass
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class ReadOnlyError(DiskFullError):
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pass
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```
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自定义异常会自动分配递增的错误码（从 100 开始），并支持继承匹配。
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### 多异常捕获
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```python
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try:
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result = operation()
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except (ValueError, TypeError):
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serial.puts("Value or Type error\n")
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```
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### 获取异常信息
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```python
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try:
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result = operation()
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except Exception as e:
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serial.puts("Error occurred\n")
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```
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## try / finally
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```python
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try:
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result = operation()
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finally:
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cleanup()
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```
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`finally` 块无论是否发生异常都会执行。
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## try / except / finally
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```python
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try:
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result = operation()
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except ValueError:
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handle_value_error()
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finally:
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cleanup()
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```
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## raise 语句
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### 抛出异常
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```python
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raise ValueError("invalid value")
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```
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### 抛出自定义异常
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```python
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raise MyCustomError("something went wrong")
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```
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### 重新抛出异常
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```python
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raise
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```
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### StopIteration
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`StopIteration` 有特殊处理，用于迭代器协议：
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```python
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raise StopIteration
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```
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## 异常处理的编译实现
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### 函数签名变换
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可能抛出异常的函数会自动添加错误输出参数：
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```python
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def risky_operation() -> t.CInt:
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if error:
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raise ValueError("bad value")
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return 0
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```
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编译后等价于：
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```c
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int risky_operation(char** __eh_msg_out__, int* __eh_code_out__) {
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if (error) {
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*__eh_code_out__ = 1; // ValueError code
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*__eh_msg_out__ = "bad value";
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return 0;
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}
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return 0;
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}
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```
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### 调用点变换
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```python
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try:
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result = risky_operation()
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except ValueError:
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handle_error()
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```
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编译后等价于：
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```c
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char* __eh_msg = NULL;
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int __eh_code = 0;
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int result = risky_operation(&__eh_msg, &__eh_code);
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if (__eh_msg != NULL) {
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if (__eh_code == 1) { // ValueError
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handle_error();
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}
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}
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```
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## 异常继承匹配
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异常匹配支持继承体系。捕获父类异常时，子类异常也会被匹配：
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```python
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class FileSystemError(Exception):
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pass
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class FileNotFoundError(FileSystemError):
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pass
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class PermissionError(FileSystemError):
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pass
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try:
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operation()
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except FileSystemError:
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# 也会捕获 FileNotFoundError 和 PermissionError
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handle_fs_error()
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```
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