1常见异常类

异常通过raise语句来触发,自定义异常形式为class SomeCustomException(Exception): pass，此外可以自己重定义异常中的方法。

>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> raise Exception("Error Info")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception: Error Info
>>>

2 捕获异常try....except

try:
    x=int(input("input  one int num:"))
    y=int(input("input divider:"))
    print("{}/{}={}".format(x,y,x/y))
except ZeroDivisionError:
    print("y can't be zero")

3 异常的传播

3.1 except子句中使用raise来传递该异常

#ValueError("Value Error") from Exception("Exception Info")
try:
    1/0
except ZeroDivisionError:
    raise 
E:\henry\Python\venv\Scripts\python.exe E:/henry/Python/Exception.py
Traceback (most recent call last):
  File "E:/henry/Python/Exception.py", line 33, in <module>
    1/0
ZeroDivisionError: division by zero

Process finished with exit code 1

3.2 except子句中使用raise AnotherException来引起别的异常

try:
    1/0
except ZeroDivisionError:
    raise ValueError("Value Error")
E:\henry\Python\venv\Scripts\python.exe E:/henry/Python/Exception.py
Traceback (most recent call last):
  File "E:/henry/Python/Exception.py", line 33, in <module>
    1/0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "E:/henry/Python/Exception.py", line 35, in <module>
    raise ValueError("Value Error")
ValueError: Value Error

3.3 raise …from …

使用raise … from None来禁用异常上下文：

try:
    1/0
except ZeroDivisionError:
    raise ValueError("Value Error") from None

E:\henry\Python\venv\Scripts\python.exe E:/henry/Python/Exception.py
Traceback (most recent call last):
  File "E:/henry/Python/Exception.py", line 35, in <module>
    raise ValueError("Value Error") from None
ValueError: Value Error

使用raise…from…来自定义异常上下文：

try:
    1/0
except ZeroDivisionError:
    raise ValueError("Value Error") from Exception("Exception Info")

E:\henry\Python\venv\Scripts\python.exe E:/henry/Python/Exception.py
Exception: Exception Info

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

Traceback (most recent call last):
  File "E:/henry/Python/Exception.py", line 35, in <module>
    raise ValueError("Value Error") from Exception("Exception Info")
ValueError: Value Error

3.4 多个except子句来处理异常

将字符串作为除数或被除数时会引发TypeError异常。

try:
    x=input("input  one int num:")
    y=input("input divider:")
    print("{}/{}={}".format(x,y,x/y))
except ZeroDivisionError:
    print("y can't be zero")
except TypeError:
    print("you should input one number ranther than one string")

3.5 使用(except1,except2,…)来同时处理多个异常

try:
    x=input("input  one int num:")
    y=input("input divider:")
    print("{}/{}={}".format(x,y,x/y))
except (ZeroDivisionError,TypeError):
    print("Error Input")

3.6 捕获对象

try:
    x=input("input  one int num:")
    y=input("input divider:")
    print("{}/{}={}".format(x,y,x/y))
except (ZeroDivisionError,TypeError) as e:
    print(e)

3.7 try...except...else

在没有出现异常时执行一个else子句

while True:
    try:
        x=int(input("Enter the first num:"))
        y=int(input("Enter the second num:"))
        val=x/y
        print("x/y is",val)
    except Exception as e:  #存在异常时会重新输入数字
        print("Error Input:",e)
        print("Try again!!!")
    else:   #没有异常时会进入此处执行
        break

3.8 finally子句

无论是否存在异常，均会执行finally子句，finally子句中适合执行一些清理工作（如关闭套接字等）。

while True:
    try:
        x=int(input("Enter the first num:"))
        y=int(input("Enter the second num:"))
        val=x/y
        print("x/y is",val)
    except Exception as e:  #存在异常时会重新输入数字
        print("Error Input:",e)
        print("Try again!!!")
    else:   #没有异常时会进入此处执行
        break
    finally:    #无论是否存在异常，均会执行finallly子句
        print("clean up")

4 警告

如果你只想发出警告，指出情况偏离了正轨，可使用模块warnings中的函数warn。

>>> from warnings import warn
>>> warn("I've got a bad feeling about this.")
__main__:1: UserWarning: I've got a bad feeling about this.
>>>

如果其他代码在使用你的模块，可使用模块warnings中的函数filterwarnings来抑制你发出的警告（或特定类型的警告），并指定要采取的措施，如"error"或"ignore"。

>>> from warnings import filterwarnings
>>> filterwarnings("ignore")
>>> warn("Anyone out there?")
>>> filterwarnings("error")
>>> warn("Something is very wrong!")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UserWarning: Something is very wrong!

如你所见，引发的异常为UserWarning。发出警告时，可指定将引发的异常（即警告类别），但必须是Warning的子类。如果将警告转换为错误，将使用你指定的异常。另外，还可根据异常来过滤掉特定类型的警告。

>>> filterwarnings("error")
>>> warn("This function is really old...", DeprecationWarning)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
DeprecationWarning: This function is really old...
>>> filterwarnings("ignore", category=DeprecationWarning)
>>> warn("Another deprecation warning.", DeprecationWarning)
>>> warn("Something else.")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UserWarning: Something else.