Python entry to proficient (6): exception handling (exception capture)

The concept of exception

When the program is running, if the Python interpreter encounters an error, it will stop the execution of the program and prompt some error messages , which is the exception

The program stops execution and prompts an error message . We usually call this action: throwing an exception

During program development, it is difficult to deal with all special cases . Through exception capture , emergency events can be processed in a centralized manner, thereby ensuring the stability and robustness of the program.

2. Python built-in exceptions

Let's first look at the exception hierarchy:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      |    +-- ModuleNotFoundError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      |    +-- RecursionError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning

Through the above exception hierarchy, we can clearly see that BaseException is the base class of all exceptions, which are divided into four types: SystemExit, KeyboardInterrupt, GeneratorExit, and Exception. Exception is the base class of all non-system exit exceptions. Python advocates inheriting Exception or its subclasses to derive new exceptions; Exception contains a variety of common exceptions such as: MemoryError (memory overflow), BlockingIOError (IO exception), SyntaxError (syntax error exception)... Details can be found below List:

abnormal	describe
BaseException	Base class for all exceptions
SystemExit	interpreter request to exit
KeyboardInterrupt	User interrupts execution (usually by typing ^C)
Exception	base class for general errors
StopIteration	iterator has no more values
GeneratorExit	Generator (generator) exception to notify exit
StandardError	Base class for all built-in standard exceptions
ArithmeticError	Base class for all numerical computation errors
FloatingPointError	floating point calculation error
OverflowError	Numerical operation exceeds maximum limit
ZeroDivisionError	divide (or modulo) zero (all data types)
AssertionError	Assertion statement failed
AttributeError	Object does not have this property
EOFError	No built-in input, reaching the EOF marker
EnvironmentError	Base class for operating system errors
IOError	I/O operation failed
OSError	operating system error
WindowsError	system call failed
ImportError	Failed to import module/object
LookupError	Base class for invalid data queries
IndexError	There is no such index in the sequence (index)
KeyError	There is no such key in the map
MemoryError	Out of memory error (not fatal to the Python interpreter)
NameError	undeclared/initialized object (no properties)
UnboundLocalError	access uninitialized local variable
ReferenceError	Weak reference attempts to access objects that have been garbage collected
RuntimeError	General runtime errors
NotImplementedError	Method not yet implemented
SyntaxError	Python syntax error
IndentationError	Indentation error
TabError	Mixing Tabs and Spaces
SystemError	General interpreter system errors
TypeError	Invalid operation on type
ValueError	Invalid parameter passed in
UnicodeError	Unicode related errors
UnicodeDecodeError	Error in Unicode decoding
UnicodeEncodeError	Error in Unicode encoding
UnicodeTranslateError	Error during Unicode conversion
Warning	base class for warnings
DeprecationWarning	Warning about deprecated features
FutureWarning	Warning about future semantic changes of constructs
OverflowWarning	Old warning about auto-promotion to long
PendingDeprecationWarning	Warning that features will be deprecated
RuntimeWarning	可疑的运行时行为(runtime behavior)的警告
SyntaxWarning	可疑的语法的警告
UserWarning	用户代码生成的警告

三、异常处理

Python 程序捕捉异常使用 try/except 语句

try:
    语句
    语句
except 异常的名称 as 变量名:     # 使用as可以将异信息保存到变量中，也可以省略as及其后面的部分
    语句
    语句
except 异常的名称:
    语句
    语句
except 异常的名称:
    语句
    语句
...
else:              # else 中的部分会在没有发生异常的情况才执行
	语句
    语句
finally:           # finally中的语句，无论是否发生异常，语句都会执行
	语句
    语句

try 语句的工作方式为：

• 首先，执行 try 子句 （在 try 和 except 关键字之间的部分）；

• 如果没有异常发生， except 子句 在 try 语句执行完毕后就被忽略了；

• 如果在 try 子句执行过程中发生了异常，那么该子句其余的部分就会被忽略；

• 如果异常匹配于 except 关键字后面指定的异常类型，就执行对应的except子句，然后继续执行 try 语句之后的代码；

• 如果发生了一个异常，在 except 子句中没有与之匹配的分支，它就会传递到上一级 try 语句中；

• 如果最终仍找不到对应的处理语句，它就成为一个 未处理异常，终止程序运行，显示提示信息。

其中，else 子句只能出现在所有 except 子句之后，只有在没有出现异常时执行；finally 子句放在最后，无论是否出现异常都会执行。 

四、抛出异常

使用 raise 语句允许强制抛出一个指定的异常，要抛出的异常由 raise 的唯一参数标识，它必需是一个异常实例或异常类（继承自 Exception 的类），如：

raise NameError('HiThere')

五、自定义异常 

正常来说，Python 提供的异常类型已经满足我们的使用了,但是有时候我们有定制性的需求，我们可以自定义异常类，继承自 Error 或 Exception 类就可以了，看个例子：

#自定义异常类 MyExc
class MyExc(Exception):  #继承Exception类
    def __init__(self, value):
        self.value = value
    def __str__(self):
        if self.value == 0:
            return '被除数不能为0'
#自定义方法
def getNum(n):
    try:
        if n == 0:
            exc = MyExc(n)
            print(exc)
        else:
            print(10 / n)
    except:
        pass
'''
1、调用 getNum(1)，输出结果为：
10.0

2、调用 getNum(0)，输出结果为：
被除数不能为0
'''

最后

异常处理用于处理程序错误之外，还有许多应用的地方。如关闭资源、平台兼容、模块导入等。这些使用都是基于对异常处理的实现和机制的理解上，以后我们再一起学习。