关于python中try。。。except的用法小结

关于python中try。。。except的用法小结

如果except子句中的类是同一类或其基类，则该子句与异常兼容（但不是相反的方式-列出派生类的except子句与基类不兼容）。例如，以下代码将按此顺序打印B，C，D：

>>> class C(B):
	pass

>>> class D(C):
	pass

>>> for cls in [B,C,D]:
	try:
		raise cls()
	except D:
		print("D")
	except C:
		print("C")
	except B:
		print("B")

		
B
C
D

发生异常时，它可能具有关联的值，也称为异常的参数。参数的存在和类型取决于异常类型。

except子句可以在异常名称后指定一个变量。变量绑定到带有存储在中的参数的异常实例 instance.args。为了方便起见，定义了异常实例， str()以便可以直接打印自变量而无需引用.args。人们也可以在引发异常之前先实例化异常，然后根据需要向其添加任何属性。

>>> try:
...     raise Exception('spam', 'eggs')
... except Exception as inst:
...     print(type(inst))    # the exception instance
...     print(inst.args)     # arguments stored in .args
...     print(inst)          # __str__ allows args to be printed directly,
...                          # but may be overridden in exception subclasses
...     x, y = inst.args     # unpack args
...     print('x =', x)
...     print('y =', y)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs

如果异常处理程序立即在try子句中发生，则异常处理程序不仅会处理异常，而且如果它们在try子句中被调用（甚至间接调用）的函数中发生，异常处理程序也不会处理异常。例如：

>>> def this_fails():
	x = 1/0
	>>> try:
		this_fails()
	except ZeroDivisionError as err:
		print('Handling run-time error:',err)

	
Handling run-time error: division by zero

抛出异常

>>> raise NameError('HiThere')
Traceback (most recent call last):
  File "", line 1, in <module>
    raise NameError('HiThere')
NameError: HiThere
>>> raise ValueError
Traceback (most recent call last):
  File "", line 1, in <module>
    raise ValueError
ValueError

>>> try:
	raise NameError('HiThere!')
except NameError:
	print('An exception flew by!')

	
An exception flew by!

如果您需要确定是否引发了异常，但又不想处理该异常，则可以使用一种更简单的raise语句形式来重新引发该异常：

>>> try:
	raise NameError('HiThere!')
except NameError:
	print('An exception flew by!')
	raise

An exception flew by!
Traceback (most recent call last):
  File "", line 2, in <module>
    raise NameError('HiThere!')
NameError: HiThere!

try语句还有另一个可选子句，用于定义必须在所有情况下都必须执行的清理操作。

>>> try:
...     raise KeyboardInterrupt
... finally:
...     print('Goodbye, world!')
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last):
  File "", line 2, in <module>

如果该try子句中发生了异常且尚未由except子句处理 （或在exceptor else子句中发生），则finally在执行该子句后将重新引发该异常。
finally当try通过break，continue或return语句留下语句的 任何其他子句时，该子句也“跳出时”执行。

>>> def divide(x, y):
...     try:
...         result = x / y
...     except ZeroDivisionError:
...         print("division by zero!")
...     else:
...         print("result is", result)
...     finally:
...         print("executing finally clause")
...
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
  File "", line 1, in <module>
  File "", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

预定义的清理动作

with open("myfile.txt") as f:
    for line in f:
        print(line, end="")

执行该语句后，即使处理行时遇到问题，文件f也始终关闭。像文件一样提供预定义清除操作的对象将在其文档中进行说明。

①https://docs.python.org/3.6/tutorial/errors.html?highlight=exceptions
②https://docs.python.org/3.6/reference/compound_stmts.html#except
③https://docs.python.org/3.6/library/exceptions.html
④https://docs.python.org/3.6/reference/simple_stmts.html#raise