Python语法速查： 10. 异常

class NetworkError(Exception):
    def __init__(self, errno, msg):
        self.args = (errno, msg)    # 赋值给args是必须的，否则用户无法看到自定义异常的任何细节提示信息
        self.errno = errno
        self.msg = msg

raise NetworkError(1, ‘No response‘)

class NetworkError(Exception): pass
    
class HostnameError(NetworkError): 
    pass
class TimeoutError(NetworkError): 
    pass

# 以下为引发并捕捉自定义异常
try:
    raise TimeoutError(‘Time out‘)
except NetworkError as e:
    if type(e) is TimeoutError:
      pass

age = -1
assert 0 < age < 100 ‘The age is fantastic.‘  # 执行本句时，会引发AssertionError异常，并会提示：The age is fantastic.

raise KeyError(‘abc‘)   # 引发KeyError异常，并提示字符串 ‘abc‘

try:
    x = 2/0
except ZeroDivisionError:
    print ‘The divisor is zero‘

# 方法一：
try:
    x = 2/‘a‘
except ZeroDivisionError:
    pass
except TypeError:
    pass
    
# 方法二：
try:
    x = 2/‘a‘
except (ZeroDivisionError, TypeError, NameError):   
    print(‘oops!‘)

try:
    x = 2/0
except Exception:   # 这里使用基类Exception可捕捉除了“键盘中断”和“程序退出”的所有异常
    pass

try:
    x = 2/0
except Exception as e:
    print(e)

try:
    f = open(‘foo‘, ‘r‘)
except IOError as e:
    print(e)
else:
    data = f.read()
    f.close()

try:
    f = open(‘foo‘, ‘r‘)
    data = f.read()    
except IOError as e:
    print(e)
finally:
    f.close()   # 无论前面发生什么，都会关闭文件

try:
    f = open(‘foo‘, ‘r‘)
except IOError as e:
    print(e)
else:
    data = f.read()
finally:
    f.close()

try:
    f = open(r‘xxxxxx‘, ‘r‘)
except Exception as e:
    print(e.errno)
    print(e.args)

# 运行结果为：
2
(2, ‘No such file or directory‘)

import os
import errno
print(os.strerror(2))

# 运行结果为错误代码2的字符串含义：No such file or directory‘

with open(‘a.txt‘, ‘w‘) as f:
    f.write(‘xyz\n‘)
    ......
    f.write(‘done\n‘)
# 当程序离开with语句块时，with语句会自动关闭已打开的文件

import threading
lock = threading.Lock()
with lock:
    ......
# 当程序离开with语句块时，with语句会自动释放这个锁

class ListTransaction(object):
    def __init__(self, thelist):
        self.thelist = thelist
    def __enter__(self):
        self.workingcopy = list(self.thelist)
        return self.workingcopy
    def __exit__(self, type, value, tb):
        if type is None:
            self.thelist[:] = self.workingcopy
        return
        
# 使用with语句：
items = [1,2,3]
try:
    with ListTransaction(items) as working:
        working.append(4)
        working.append(5)
        raise RuntimeError()
except RuntimeError:
    pass
print(items)

# 由于在离开with语句块时发生了异常，因此__exit__()的入参type不为None，最终结果为：[1,2,3]