Python基础教程之第8章 异常

#Chapter 8 异常
Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
#8.1 什么是异常
>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

#8.2 按自己的方式出错
#8.2.1 raise语句
>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception
>>> raise Exception('hyperdrive overload')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception: hyperdrive overload
>>> import exceptions
>>> dir(exceptions)
['ArithmeticError', 'AssertionError', 'AttributeError', ...]
>>> raise ArithmeticError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ArithmeticError

#8.2.2 自定义异常类
>>> class SomeCustomException(Exception): pass
...

#8.3 捕捉异常

try:
	x=input('Enter the first number: ')
	y=input('Enter the second number: ')
	print x/y

except ZeroDivisionError:
	print "The second number can't be zero!"


#打开或关闭屏蔽
class MuffleCalculator:
	muffled = False
	def calc(self, expr):
		try:
			return eval(expr)
		except ZeroDivisionError:
			if self.muffled:
				print 'Division by zero is illegal'
			else:
				raise
				
calculator=MuffleCalculator()
print calculator.calc('10/2')
#print calculator.calc('10/0') # No muffling
#5
#Traceback (most recent call last):
#  File "tta.py", line 14, in <module>
#    print calculator.calc('10/0') # No muffling
#  File "tta.py", line 5, in calc
#    return eval(expr)
#  File "<string>", line 1, in <module>
#ZeroDivisionError: integer division or modulo by zero

calculator.muffled=True
calculator.calc('10/0')
#Division by zero is illegal

#8.4 不止一个 except 语句
try:
	x=input('Enter the first number: ')
	y=input('Enter the second number: ')
	print x/y
except ZeroDivisionError:
	print "The second number can't be zero!"
except TypeError:
	print "That wasn't a number, was it?"

	#8.5 用一个块捕捉两个异常
try:
	x=input('Enter the first number: ')
	y=input('Enter the second number: ')
	print x/y

except (ZeroDivisionError, TypeError, NameError):
	print 'Your numbers are bogus...'

#8.6 捕捉对象
try:
	x=input('Enter the first number: ')
	y=input('Enter the second number: ')
	print x/y

except (ZeroDivisionError, TypeError, NameError), e:
	print e

#在Python 3.0 中, except子句会被写作 except (ZeroDivisionError, TypeError, NameError) as e

#8.7 真正的全捕捉
try:
	x=input('Enter the first number: ')
	y=input('Enter the second number: ')
	print x/y

except:
	print 'Something wrong happened...'


>>> x=input('Enter the first number: ')
Enter the first number: 10
>>> y=input('Enter the second number: ')
Enter the second number: 0
>>> print x/y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

#8.8 万事大吉
>>> try:
...     print 'A simple task'
... except:
...     print 'What? Something went wrong?'
... else:
...     print 'Ah... It went as planned.'
...
A simple task
Ah... It went as planned.

#只要有错误发生, 程序会不断要求重新输入
while True:
	try:
		x=input('Enter the first number: ')
		y=input('Enter the second number: ')
		value = x/y
		print 'x/y is', value
	except:
		print 'Invalid input. Please try again.'
	else:
		break

#python ttb.py
#Enter the first number: 1
#Enter the second number: 0
#Invalid input. Please try again.
#Enter the first number: 'foo'
#Enter the second number: 'bar'
#Invalid input. Please try again.
#Enter the first number: baz
#Invalid input. Please try again.
#Enter the first number: 10
#Enter the second number: 2
#x/y is 5

#打印更加有用的错误信息:

while True:
	try:
		x=input('Enter the first number: ')
		y=input('Enter the second number: ')
		value = x/y
		print 'x/y is', value
	except Exception, e:
		print 'Invalid input:', e
		print 'Please try again'
	else:
		break
#python ttc.py
#Enter the first number: 1
#Enter the second number: 0
#Invalid input: integer division or modulo by zero
#Please try again
#Enter the first number: 'x'
#Enter the second number: 'y'
#Invalid input: unsupported operand type(s) for /: 'str' and 'str'
#Please try again
#Enter the first number: quuux
#Invalid input: name 'quuux' is not defined
#Please try again
#Enter the first number: 10
#Enter the second number: 2
#x/y is 5

#8.8 最后

>>> x=None
>>> try:
...     x = 1/0
... finally:
...     print 'Cleaning up...'
...     del x
...
Cleaning up...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero
>>>
>>> try:
...     1/0
... except NameError:
...     print 'Unknown variable'
... else:
...     print 'That went well!'
... finally:
...     print 'Cleaning up.'
...
Cleaning up.
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero

#异常和函数
>>> def faulty():
...     raise Exception('Someting is wrong')
...
>>> def ignore_exception():
...     faulty()
...
>>> def handle_exception():
...     try:
...             faulty()
...     except:
...             print 'Exception handled'
...
>>>

>>> ignore_exception()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in ignore_exception
  File "<stdin>", line 2, in faulty
Exception: Something is wrong
>>>
>>> handle_exception()
Exception handled
>>>
#8.11 异常之禅

>>> def describePerson(person):
...     print 'Description of', person['name']
...     print 'Age:', person['age']
...     if 'occupation' in person:
...             print 'Occupation:', person['occupation']
...
def describePerson(person):
	print 'Description of', person['name']
	print 'Age:', person['age']
	try:
		print 'Occupation: ' + person['occupation']
	except KeyError: pass
	
person = {'name':'Jonathan', 'age':33}
describePerson(person)
#python ttd.py
#Description of Jonathan
#Age: 33

person = {'name':'Jonathan', 'age':33, 'occupation':'IBIT Support'}
describePerson(person)
#python ttd.py
#Description of Jonathan
#Age: 33
#Occupation: IBIT Support

#8.12 小结
# 异常对象
# 引发异常
# 自定义异常类
# 捕捉异常: 使用try语句的except子句捕捉异常. 如果在except子句中不特别指定异常类, 那么所有的异常都会被捕捉.
# 异常可以放在 元组 中以实现多个异常的指定.
# 如果给except提供两个参数, 第二个参数就会绑定到 异常对象 上
# 在一个try语句中,可以包含多个except子句, 用来分别处理不同的异常

#else子句 如果主块try中没有发生异常, else子句就会被执行
#finally子句-- 如果需要确保某些代码不管是否有异常引发都要执行(比如清理代码, 关闭文件, 关闭网络套接字, 关闭数据库连接),
# 那么这些代码可以放置在finally子句中

#异常和函数 在函数内引发异常时, 它就会被传播到函数调用的地方(对于方法也是一样)

#8.12.1 新函数
# warnings.filterwarning(action....)		用于过滤警告

#Exception	所有异常的基类
#AttributeError		特性引用或赋值失败时引发
#IOError		试图打开不存在的文件(包括其他情况)时引发
#IndexError		在使用序列中不存在的索引时引发
#KeyError		在使用映射中不存在的键时引发
#NameError		在找不到名字(变量)时引发
#SyntaxError	在代码为错误形式时引发
#TypeError		在内建操作或函数应用于错误类型的对象时引发
#ValueError		在内建操作或函数应用于正确类型的对象,但该对象使用不合适的值时引发
#ZeroDivisionError		在除法或模除操作的第二个参数为0时引发