Python语法 - raise ... from 用法

前言

当程序出现错误时,系统会自动触发异常。Python 也允许程序自行引发异常,自行引发异常使用 raise 语句来完成。

  • 使用 raise 抛出新的异常
  • 使用 raise ... from ... 抛出新的异常时,新的异常是由旧的异常表现的;
  • 使用 raise ... from None 抛出新的异常时,不会打印旧的异常(即禁止的异常关联)

raise 引发异常

使用 raise 语句,主动引发异常,终止程序

x = 20

if not isinstance(x, str):
    raise Exception("value is not type of str")
else:
    print("hello")

运行结果:

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 4, in <module>
    raise Exception("value is not type of str")
Exception: value is not type of str

当raise 抛出异常时,程序就会终止

try... except 捕获异常

使用try... except 捕获异常

x = [20, 3, 22, 11]

try:
    print(x[7])
except IndexError:
    print("index out of list")

运行后不会有异常
在捕获异常后,也可以重新抛一个其它的异常

x = [20, 3, 22, 11]

try:
    print(x[7])
except IndexError:
    print("index out of list")
    raise NameError("new exception ...")

输出结果:

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 4, in <module>
    print(x[7])
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 7, in <module>
    raise NameError("new exception ...")
NameError: new exception ...

在抛出异常的日志中,可以看到日志中对 IndexError 和 NameError之间是描述是 During handling of the above exception, another exception occurred,即在处理 IndexError 异常时又出现了 NameError 异常,两个异常之间没有因果关系。

raise ... from 用法

示例:

x = [20, 3, 22, 11]

try:
    print(x[7])
except IndexError as e:
    print("index out of list")
    raise NameError("new exception ...") from e

运行结果

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 4, in <module>
    print(x[7])
IndexError: list index out of range

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 7, in <module>
    raise NameError("new exception ...") from e
NameError: new exception ...

在抛出异常的日志中,可以看到日志中对 IndexError和 NameError 之间是描述是 The above exception was the direct cause of the following exception,即因为 IndexError 直接异常导致了 NameError异常,两个异常之间有直接因果关系。

示例:

x = [20, 3, 22, 11]

try:
    print(x[7])
except IndexError as e:
    print("index out of list")
    raise NameError("new exception ...") from None

运行结果

Traceback (most recent call last):
  File "D:/demo/untitled1/demo/a.py", line 7, in <module>
    raise NameError("new exception ...") from None
NameError: new exception ...

在抛出异常的日志中,可以看到日志只打印了 NameError 而没有打印 IndexError

posted @ 2024-05-28 14:44  上海-悠悠  阅读(217)  评论(0编辑  收藏  举报