Chapter 9 Exceptions 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,The program will terminate with an error message（traceback）.

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

Exceptions include built-in exceptions and custom exceptions2种（没有本质区别,Built-in exceptions iePythonExceptions that are already defined and used frequently,Custom exceptions are mostly related to the actual business development）

内置异常

Common built-in exceptions only,其实还有很多

BaseException                                 
+-- Exception                               
|    +-- AttributeError		引用属性或给它赋值失败时引发                   
|    +-- OSError             Raised when the operating system cannot perform the specified task                
|    +-- IndexError			Raised when an index does not exist in the used sequence
|    +-- KeyError			Raised when using a key that does not exist in the map
|    +-- NameError			找不到名称（变量）时引发
|    +-- SyntaxError		代码不正确时引发
|    +-- TypeError			Raised when a built-in operation or function is used with a variable of the wrong type
|    +-- 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 The location is triggeredZeroDivisionError,raiseThrows the entire exception stack
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 The location is triggeredZeroDivisionError,except ZeroDivisionErrorException handling was thrownValueError,except Exception通过raiseThrows the entire exception stack
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,raiseThe entire exception stack is not thrown but fromValueErrorBegin the outgoing exception stack
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

# tryblock required,except块&finallyChoose one of two blocks,else块可选
# try块中的代码出现异常,终止tryThe execution of the code following the block is transferred inexcept块
# tryThe code in the block does not throw an exception,执行完tryTransfer in after the blockelse块
# tryThe code in the block regardless of whether an exception occurs,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:

# Catch an exception
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 ...')
    
# Access the exception object itself
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,The program continues to execute the following statement
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exceptAfter the processing of the block,The program continues to execute the following statement
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,Handed over to the outer caller to handle
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filterwarningsThe function suppresses warnings（或特定类型的警告）,and specify the measures to be taken,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,You can specify the warning category to raise（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...
#Filter specific types of warnings based on exceptions
>>>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.