識(shí)別異常(錯(cuò)誤)

Python中發(fā)生的每個(gè)錯(cuò)誤都會(huì)導(dǎo)致異常，這個(gè)異常將由其錯(cuò)誤類型識(shí)別出錯(cuò)誤條件。

>>> #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.

捕獲異常

當(dāng)程序中出現(xiàn)異常并且您希望使用異常機(jī)制處理它時(shí)，可能需要“拋出異?！?。 關(guān)鍵字try和except用于捕獲異常。 每當(dāng)try塊中發(fā)生錯(cuò)誤時(shí)，Python都會(huì)查找匹配的except塊來(lái)處理它。 如果有，執(zhí)行跳轉(zhuǎn)到那里。

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

try子句中的代碼將逐語(yǔ)句執(zhí)行。
如果發(fā)生異常，將跳過(guò)try塊的其余部分并執(zhí)行except子句。

try:
   some statement here
except:
   exception handling

下面編寫(xiě)一些代碼，看看在程序中不使用任何錯(cuò)誤處理機(jī)制時(shí)會(huì)發(fā)生什么。

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

只要用戶輸入一個(gè)數(shù)字，上面的程序就能正常工作，但如果用戶試圖放入一些其他數(shù)據(jù)類型(如字符串或列表)會(huì)發(fā)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'"

現(xiàn)在ValueError是一種異常類型。下面嘗試用異常處理重寫(xiě)上面的代碼。

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)

如果運(yùn)行上面程序，并輸入一個(gè)字符串(而不是數(shù)字)，應(yīng)該會(huì)看到不同的結(jié)果。

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

引發(fā)異常

要從您自己的方法中引發(fā)異常，需要使用raise關(guān)鍵字 -

raise ExceptionClass('Some Text Here')

看看下面一個(gè)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')

運(yùn)行程序并輸入正整數(shù)，應(yīng)該會(huì)得到類似以下的結(jié)果 -

Enter your age: 12
Entered Age is even

但是當(dāng)輸入負(fù)數(shù)時(shí)，得到以下結(jié)果 -

Enter your age: -2
Only positive integers are allowed

創(chuàng)建自定義異常類

可以通過(guò)擴(kuò)展BaseException類或BaseException的子類來(lái)創(chuàng)建自定義異常類。

從上圖中可以看到Python中的大多數(shù)異常類都是從BaseException類擴(kuò)展而來(lái)的。可以從BaseException類或其子類派生自己的異常類。

創(chuàng)建一個(gè)名為NegativeNumberException.py的新文件并編寫(xiě)以下代碼。

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

上面的代碼創(chuàng)建了一個(gè)名為NegativeNumberException的新異常類，它只包含使用super().__ init __()調(diào)用父類構(gòu)造函數(shù)的構(gòu)造函數(shù)，并設(shè)置年齡。

現(xiàn)在要?jiǎng)?chuàng)建自己的自定義異常類，將編寫(xiě)一些代碼并導(dǎo)入新的異常類。

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')

執(zhí)行上面示例代碼，得到以下結(jié)果 -

Enter your age: -2
Only positive integers are allowed

另一種創(chuàng)建自定義Exception類的方法。

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)

執(zhí)行上面示例代碼，得到以下結(jié)果 -

Caught: My Useful Error Message!

異常層次結(jié)構(gòu)
內(nèi)置異常的類層次結(jié)構(gòu)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