第9章 异常try...except...else...finally

异常

Python使用异常对象来表示异常状态，并在遇到错误时引发异常。异常对象未被处理（或捕获）时，程序将终止并显示一条错误信息（traceback）。

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

异常包含内置异常和自定义异常2种（没有本质区别，内置异常即Python已经定义且经常使用的异常，自定义异常则多与实际开发的业务有关）

内置异常

仅常见的内置异常，其实还有很多

BaseException                                 
+-- Exception                               
|    +-- AttributeError		引用属性或给它赋值失败时引发                   
|    +-- OSError             操作系统不能执行指定的任务时引发                
|    +-- IndexError			使用序列中不存在索引时引发
|    +-- KeyError			使用映射中不存在键时引发
|    +-- NameError			找不到名称（变量）时引发
|    +-- SyntaxError		代码不正确时引发
|    +-- TypeError			将内置操作或函数用于类型不正确的变量时引发
|    +-- ValueError          将内置操作或函数用于这样的对象时引发：其类型正确但包含的值不合适
|    +-- ZeroDivisionError	 在除法或求模运算的第二个参数为零时引发
+-- KeyboardInterrupt                      
+-- SystemExit                               

自定义异常

class NewException(Exception):
    def __init__(self, errorInfo):
        super().__init__()
        self.errorInfo = errorInfo

    def __str__(self):
        return self.errorInfo

抛出异常

1） raise：单独一个 raise。该语句引发当前上下文中捕获的异常（比如在 except 块中），或默认引发 RuntimeError 异常
注意：raise ... from None的用法

# 引发默认的RuntuimeError
raise
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 1, in <module>
# raise 
# RuntimeError: No active exception to reraise

# 1 / 0 的位置引发了ZeroDivisionError，raise将整个异常栈抛出
try:
    1 / 0
except ZeroDivisionError:
    raise
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 2, in <module>
# 1 / 0
# ZeroDivisionError: division by zero

# 1 / 0 的位置引发了ZeroDivisionError，except ZeroDivisionError异常处理抛出了ValueError，except Exception通过raise将整个异常栈抛出
try:
    try:
        1 / 0
    except ZeroDivisionError:
        raise ValueError
except Exception:
    raise    
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 3, in <module>
# 1 / 0
# ZeroDivisionError: division by zero
# 
# During handling of the above exception, another exception occurred:
# 
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 5, in <module>
# raise ValueError
# ValueError

# 通过raise ... from None来禁用上下文，raise并没有抛出整个异常栈而是从ValueError开始向外的异常栈
try:
    try:
        1 / 0
    except ZeroDivisionError:
        raise ValueError from None
except Exception:
    raise
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 5, in <module>
# raise ValueError from None
# ValueError

2） raise ZeroDivisionError
# Traceback (most recent call last):
# File "E:\anaconda\envs\py37tfcpu\lib\code.py", line 90, in runcode
# exec(code, self.locals)
# File "<input>", line 1, in <module>
# ZeroDivisionError

3） raise ZeroDivisionError("除数不能为0")
# Traceback (most recent call last):
# File "E:\anaconda\envs\py37tfcpu\lib\code.py", line 90, in runcode
# exec(code, self.locals)
# File "<input>", line 1, in <module>
# ZeroDivisionError: 除数不能为0

捕获异常

总体结构

# try块必选，except块&finally块二选一，else块可选
# try块中的代码出现异常，终止try块后面的代码执行转入except块
# try块中的代码未出现异常，执行完try块后转入else块
# try块中的代码无论是否出现异常，最后执行finally块
try:
    pass
except:
    pass
else:
    pass
finally:
    pass

except块

# 捕获所有异常，不建议使用，因为包含KeyboardInterrupt等
except:

# 捕获一种异常
except ZeroDivisionError:
    print("The second number can't bu zero!")

# 捕获多种异常
except ZeroDivisionError:
    print("The second number can't bu zero!")
except TypeError:
    print("That wasn't a number, was it?")

# 捕获多种异常
except (ZeroDivisionError, TypeError):
    print('Your numbers were bogus ...')
    
# 访问异常对象本身
except (ZeroDivisionError, TypeError) as e:
    print(e)

处理异常

1）不处理，程序将异常终止
1 / 0
print('Hello World')
# Traceback (most recent call last):
# File "C:/Users/shang/Desktop/pythonProject/main.py", line 1, in <module>
# 1 / 0
# ZeroDivisionError: division by zero
# 
# Process finished with exit code 1

2）忽略，程序继续执行后面的语句
try:
    1 / 0
except Exception as e:
    pass
print('Hello World')
# Hello World
# 
# Process finished with exit code 0

3）处理，执行完except块的处理过程后，程序继续执行后面的语句
try:
    1 / 0
except Exception as e:
    print(e)
print('Hello World')
# division by zero
# Hello World
# 
# Process finished with exit code 0

4）抛出，交由外层的调用者处理
try:
    1 / 0
except Exception as e:
    raise 
print('Hello World')

警告

#使用warnings模块中的warn函数warn发出警告
>>>from warnings import warn
>>>warn("I've got a bad feeling about this.")
#警告只显示一次，如果再次运行最后一行代码，什么事情都不会发生
 
#使用warnings模块中的filterwarnings函数抑制警告（或特定类型的警告），并指定采取的措施，如"error"（警告）或"ignore"（忽略警告）
>>>from  warnings import filterwarnings
 
>>>filterwarnings("ignore")
>>>warn("Anyone out there?")
 
>>>filterwarnings("error")
>>>warn("Something is very wrong!")
Traceback (most recent call last):
  File "<input>", line 1, in <module>
UserWarning: Something is very wrong!
 
#发出警告时，可以指定要引发的警告类别（必须是Warning的子类，默认UserWarning）。
>>>filterwarnings("error")
>>>warn("This function is really old...", DeprecationWarning)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
DeprecationWarning: This function is really old...
#根据异常来过滤特定类型的警告
>>>filterwarnings("ignore", category=DeprecationWarning)
>>>warn("Another deprecation warning.", DeprecationWarning)
>>>warn("Something else.")
Traceback (most recent call last):
  File "<input>", line 1, in <module>
UserWarning: Something else.