Python，异常 exception

同Java一样，Pyton异常对象来表示异常情况。遇到错误后，引发异常，如果异常对象并未对处理或捕捉，程序就会用所谓的回溯（Traceback）来终止执行；

>>> 1/0

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ZeroDivisionError: division by zero



程序可以通过raise Exceptin来手动抛出异常

>>> raise Exception

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

Exception

>>> raise Exception("hello exception")

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

Exception: hello exception



自定义异常类型，只需要继承Exception（或其子类）即可；

>>> raise 1

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: exceptions must derive from BaseException



不同于C++，只有异常类才能被抛出；

>>> class MyException(Exception):

... pass

...

>>> raise MyException

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

__main__.MyException



捕获异常





try:

….

except ExceptionType1 as e:

….

except ExceptionType2:

….

raise

except (ExceptionType3, ExceptionType4):

….

except:

…

…

else:

…



finally:

…



不带参数的raise用于重新抛出异常；exception (ExceptionType3, ExceptionType4)相比与Java的单参数方式更方便了；



不带参数的except捕捉所有类型的异常，但是得不到异常对象，可以使用except BaseException as e 解决（或者Exceptin）

>>> try:

... 1/0

... except BaseException as e:

... print(e)

...

division by zero





else的块在没有引发异常时执行；finally同java；



>>> try:

... 1/0

... except ZeroDivisionError as e:

... print(e)

...

division by zero



>>> try:

... 1/0

... finally:

... print('finally')

...

finally

Traceback (most recent call last):

File "<stdin>", line 2, in <module>

ZeroDivisionError: division by zero



>>> def f():

... try:

... return 1/0

... finally:

... return 2

...

>>> f()

2