Python学习笔记6-异常

异常

raise

raise语句以Exception子类为参数, 抛出异常:

​

>>> raise Exception("Overload")
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    raise Exception("Overload")
Exception: Overload

一些内置的异常类:

自定义异常类:

# 从Exception中直接或间接派生
>>> class MyException(TypeError): pass

>>> raise MyException
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    raise MyException
MyException

捕获异常(P134)

​

try:
    1/0
except ZeroDivisionError:
    print("ZeroDivisionError")

使用异常机制代替if检查会更方便

捕获异常后, 如果要使它向上传播可调用raise且不提供任何参数

进入except的异常会作为异常上下文存储起来, 并且会出现在最终的消息中, 如果想要自定义异常上下文, 可以使用raise ... from ...语句, 使用raise ... from ...可以禁用异常上下文

# 异常上下文
try: 1/0
except ZeroDivisionError: raise ValueError

Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    try: 1/0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<pyshell#12>", line 2, in <module>
    except ZeroDivisionError: raise ValueError
ValueError

# 自定义异常上下文
>>> raise ZeroDivisionError from TypeError
TypeError

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

Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    raise ZeroDivisionError from TypeError
ZeroDivisionError

可以使用一个except语句捕获多个异常, 用as语句获得捕获的异常对象:

except (NameError, ZeroDivisionError) as e: print(e)

使用不带任何参数的except语句捕获全部异常:

try :
	x = input()
except:
	pass
# 将捕获所有异常, 包括用户使用Ctrl+C终止程序的企图

try/except语句中的else子句会在没有出现异常是执行:

try: print("Hello")
except: print("Wrong")
else: print("Right")

Hello
Right

finally子句与try配套, 用于在发生异常后进行处理工作, 无论发生什么异常都会执行finally子句, 可以用于确保文件或网络套接字得以关闭:

try: 1/0
except Exception as e: print(e)
finally: print("finally")

division by zero
finally

警告(P142)

>>> from warnings import warn
>>> warn("123")

Warning (from warnings module):
  File "C:\Users\Lee\Desktop\test.py", line 1
    try:
UserWarning: 123
>>> warn("123")
>>>

# 警告消息只显示一次