异常和异常类

一般来说,异常是任何不寻常的情况。异常通常表示错误,但有时它们是故意放入程序中的,例如提前终止程序或从资源短缺中恢复。有许多内置异常,它们表示诸如读取超过文件末尾或除以零之类的情况。我们可以定义自己的异常,称为自定义异常。

异常处理使您能够优雅地处理错误并对其采取有意义的措施。异常处理有两个组成部分:"抛出"和"捕获"。

识别异常(错误)

Python 中发生的每个错误都会导致异常,该异常将由其错误类型标识为错误条件。

>>> #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.

捕获/捕捉异常

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,执行会跳转到那里。

语法

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

try 子句内的代码将逐条执行。

如果发生异常,则将跳过 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来看看当你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)

只要用户输入数字,上述程序就能正常工作,但如果用户尝试输入其他类型的数据类型(如字符串或列表),会发生什么情况呢?

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。让我们尝试使用异常处理重写上述代码。

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

引发异常

要从您自己的方法中引发异常,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

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。

预期输出

Enter your age: 12
Entered Age is even

但是当我们尝试输入负数时,我们得到的是,

预期输出

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。

自定义异常类

从上图我们可以看到 Python 中的大多数异常类都从 BaseException 类扩展而来。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

以上代码创建了一个名为 NegativeNumberException 的新异常类,该类仅包含构造函数,该构造函数使用 super()__init__() 调用父类构造函数并设置年龄。

现在要创建您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

Enter your age: -2
Only positive integers are allowed

创建自定义异常类的另一种方法。

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

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