1.try-except-else-finally
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error: {0}".format(err))
except ValueError:
print("Could not convert data to an integer.")
except:
print("Unexpected error:", sys.exc_info()[0])
raise
else必须放在所有的except子句之后，在try子句没有发生任何异常的时候执行。
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
2.抛出异常
raise 语句抛出一个指定的异常。抛出的异常必须是一个异常的实例或者是异常的类，也就是 Exception 的子类。
>>>raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: HiThere
In [56]: sys.exc_info?
Docstring:
exc_info() -> (type, value, traceback)
Return information about the most recent exception caught by an except
clause in the current stack frame or in an older stack frame.
Type: builtin_function_or_method
3.自定义异常
异常类继承自 Exception 类，可以直接继承，或者间接继承。
>>>class MyError(Exception):
def __init__(self, value):
self.value = value
def __str__(self):
return repr(self.value)
>>> try:
raise MyError(2*2)
except MyError as e:
print('My exception occurred, value:', e.value)
My exception occurred, value: 4
>>> raise MyError('oops!')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'
类 Exception 默认的 init() 被覆盖。当创建一个模块有可能抛出多种不同的异常时，一种通常的做法是为这个包建立一个基础异常类，然后基于这个基础类为不同的错误情况创建不同的子。
class Error(Exception):
"""Base class for exceptions in this module."""
pass
class InputError(Error):
"""Exception raised for errors in the input.
Attributes:
expression -- input expression in which the error occurred
message -- explanation of the error
"""
def __init__(self, expression, message):
self.expression = expression
self.message = message
class TransitionError(Error):
"""Raised when an operation attempts a state transition that's not
allowed.
Attributes:
previous -- state at beginning of transition
next -- attempted new state
message -- explanation of why the specific transition is not allowed
"""
def __init__(self, previous, next, message):
self.previous = previous
self.next = next
self.message = message
————Blueicex 2020/2/22 22:38 [email protected]
