异常和异常类（Exception and Exception Classes）

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2023-12-01

通常，例外是任何异常情况。例外通常表示错误，但有时他们故意放入程序，例如提前终止程序或从资源短缺中恢复。有许多内置异常，表示读取文件末尾或除以零等条件。我们可以定义自己的异常，称为自定义异常。

异常处理使您可以优雅地处理错误并对其执行有意义的操作。异常处理有两个组成部分:“抛出”和“捕获”。

Identifying Exception (Errors)

Python中发生的每个错误都会导致异常，这个异常将由其错误类型识别出错误条件。

>>> #Exception
>>> 1/0
Traceback (most recent call last):
   File "<pyshell#2>", line 1, in <module>
      1/0
ZeroDivisionError: division by zero
>>>
>>> var = 20
>>> print(ver)
Traceback (most recent call last):
   File "<pyshell#5>", line 1, in <module>
      print(ver)
NameError: name 'ver' is not defined
>>> #Above as we have misspelled a variable name so we get an NameError.
>>>
>>> print('hello)
SyntaxError: EOL while scanning string literal
>>> #Above we have not closed the quote in a string, so we get SyntaxError.
>>>
>>> #Below we are asking for a key, that doen't exists.
>>> mydict = {}
>>> mydict['x']
Traceback (most recent call last):
   File "<pyshell#15>", line 1, in <module>
      mydict['x']
KeyError: 'x'
>>> #Above keyError
>>>
>>> #Below asking for a index that didn't exist in a list.
>>> mylist = [1,2,3,4]
>>> mylist[5]
Traceback (most recent call last):
   File "<pyshell#20>", line 1, in <module>
      mylist[5]
IndexError: list index out of range
>>> #Above, index out of range, raised IndexError.

Catching/Trapping Exception

当程序中出现异常并且您希望使用异常机制处理它时，您会“抛出异常”。关键字try和except用于捕获异常。每当try块中发生错误时，Python都会查找匹配的except块来处理它。如果有，执行跳转到那里。

句法

try:
   #write some code
   #that might throw some exception
except <ExceptionType>:
   # Exception handler, alert the user

try子句中的代码将逐语句执行。

如果发生异常，将跳过try块的其余部分并执行except子句。

try:
   some statement here
except:
   exception handling

让我们编写一些代码，看看在程序中不使用任何错误处理机制时会发生什么。

number = int(input('Please enter the number between 1 & 10: '))
print('You have entered number',number)

只要用户输入一个数字，上面的程序就能正常工作，但是如果用户试图放入一些其他数据类型（如字符串或列表）会发生什么。

Please enter the number between 1 > 10: 'Hi'
Traceback (most recent call last):
   File "C:/Python/Python361/exception2.py", line 1, in <module>
      number = int(input('Please enter the number between 1 & 10: '))
ValueError: invalid literal for int() with base 10: "'Hi'"

现在ValueError是一种异常类型。让我们尝试用异常处理重写上面的代码。

import sys
print('Previous code with exception handling')
try:
   number = int(input('Enter number between 1 > 10: '))
except(ValueError):
   print('Error..numbers only')
   sys.exit()
print('You have entered number: ',number)

如果我们运行程序，并输入一个字符串（而不是数字），我们可以看到我们得到了不同的结果。

Previous code with exception handling
Enter number between 1 > 10: 'Hi'
Error..numbers only

提高例外

要从您自己的方法中提出异常，您需要使用像这样的raise关键字

raise ExceptionClass(‘Some Text Here’)

我们来举个例子吧

def enterAge(age):
   if age<0:
      raise ValueError('Only positive integers are allowed')
   if age % 2 ==0:
      print('Entered Age is even')
   else:
      print('Entered Age is odd')
try:
   num = int(input('Enter your age: '))
   enterAge(num)
except ValueError:
   print('Only positive integers are allowed')

运行程序并输入正整数。

预期产出

Enter your age: 12
Entered Age is even

但是当我们尝试输入负数时，我们得到了

预期产出

Enter your age: -2
Only positive integers are allowed

创建自定义异常类

您可以通过扩展BaseException类或BaseException的子类来创建自定义异常类。

从上图中我们可以看到Python中的大多数异常类都是从BaseException类扩展而来的。您可以从BaseException类或其子类派生自己的异常类。

创建一个名为NegativeNumberException.py的新文件并编写以下代码。

class NegativeNumberException(RuntimeError):
   def __init__(self, age):
      super().__init__()
      self.age = age

上面的代码创建了一个名为NegativeNumberException的新异常类，它只包含使用super（）__ init __（）调用父类构造函数的构造函数，并设置年龄。

现在要创建自己的自定义异常类，将编写一些代码并导入新的异常类。

from NegativeNumberException import NegativeNumberException
def enterage(age):
   if age < 0:
      raise NegativeNumberException('Only positive integers are allowed')
   if age % 2 == 0:
      print('Age is Even')
   else:
      print('Age is Odd')
try:
   num = int(input('Enter your age: '))
   enterage(num)
except NegativeNumberException:
   print('Only positive integers are allowed')
except:
   print('Something is wrong')

输出 (Output)

Enter your age: -2
Only positive integers are allowed

另一种创建自定义Exception类的方法。

class customException(Exception):
   def __init__(self, value):
      self.parameter = value
   def __str__(self):
      return repr(self.parameter)
try:
   raise customException('My Useful Error Message!')
except customException as instance:
   print('Caught: ' + instance.parameter)

输出 (Output)

Caught: My Useful Error Message!

异常层次结构

内置异常的类层次结构是 -

+-- SystemExit 
+-- KeyboardInterrupt 
+-- GeneratorExit 
+-- Exception 
+-- StopIteration 
+-- StopAsyncIteration 
+-- ArithmeticError 
| +-- FloatingPointError 
| +-- OverflowError 
| +-- ZeroDivisionError 
+-- AssertionError 
+-- AttributeError 
+-- BufferError 
+-- EOFError 
+-- ImportError 
+-- 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