python中的异常类型处理

3、异常和函数

在函数内引发异常时，它就会被传播到到函数调用的地方（对于方法也是一样的）

>>> def faulty():
	raise Exception('Something is wrong')

>>> def ignore_expection():
	faulty()

	
>>> def handle_excepton():
	try:
		faulty()
	except:
		print 'Exception handled'

		
>>> ignore_expection()

Traceback (most recent call last):
  File "<pyshell#252>", line 1, in <module>
    ignore_expection()
  File "<pyshell#245>", line 2, in ignore_expection
    faulty()
  File "<pyshell#242>", line 2, in faulty
    raise Exception('Something is wrong')
Exception: Something is wrong

可以看到，faulty中产生的异常通过faulty和ignore_exception传播，最终导致了堆栈跟踪。同样地，它也传播了handle_exception，但在这个函数中被try/except语句处理。

>>> def describePerson(person):
	print 'Description of',person['name']
	print 'Age:',person['age']
	try:
		print 'Occupation:',person['occupation']
	except KeyError:
		pass

	
>>> describePerson({'name':'Throatwobbler','age':42})
Description of Throatwobbler
Age: 42
Occupation:
>>>

>>> def describePerson(person):
	print 'Description of',person['name']
	print 'Age:',person['age']
	try:
		print 'Occupation:'+ person['occupation']
	except KeyError: #是捕获键值不存在时
		pass

	
>>> describePerson({'name':'Throatwobbler','age':42})
Description of Throatwobbler
Age: 42
>>>

注：这里在打印职业时，使用加号而不是逗号。否则字符串'Occupaton:'在异常引发之前就会被输出。



查看某对象是否有write属性

try:
	object.write
except AttributeError:
	print  'The object is not writeable'
else:
	print 'The object is writeable'