本文主要是介绍学习Python 十二 （异常），对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

14.异常

14.1 什么是异常

异常：正常的情况，运行程序的过程中出现问题，BUG（错误不一定是异常）

14.2 处理异常

try代码块

try:
	#放的是有可能造成异常的代码
except:
	#处理异常

如：

try:
    num = int(input('输入一个数:'))
    result= num+11

except Exception as e:
    print('出现异常')
    print('在这里处理异常',e)
    num=int(input('重新输入一个数:'))
    result=num+11

print(result)
print('结束')

常见的异常：
import builtins
dir(builtins)

[‘ArithmeticError’, ‘AssertionError’, ‘AttributeError’, ‘BaseException’, ‘BlockingIOError’,
‘BrokenPipeError’, ‘BufferError’, ‘BytesWarning’, ‘ChildProcessError’, ‘ConnectionAbortedError’,
‘ConnectionError’, ‘ConnectionRefusedError’, ‘ConnectionResetError’, ‘DeprecationWarning’,
‘EOFError’, ‘Ellipsis’, ‘EnvironmentError’, ‘Exception’, ‘False’, ‘FileExistsError’, ‘FileNotFoundError’,
‘FloatingPointError’, ‘FutureWarning’, ‘GeneratorExit’, ‘IOError’, ‘ImportError’, ‘ImportWarning’,
‘IndentationError’, ‘IndexError’, ‘InterruptedError’, ‘IsADirectoryError’, ‘KeyError’,
‘KeyboardInterrupt’, ‘LookupError’, ‘MemoryError’, ‘ModuleNotFoundError’, ‘NameError’, ‘None’,
‘NotADirectoryError’, ‘NotImplemented’, ‘NotImplementedError’, ‘OSError’, ‘OverflowError’,
‘PendingDeprecationWarning’, ‘PermissionError’, ‘ProcessLookupError’, ‘RecursionError’,
‘ReferenceError’, ‘ResourceWarning’, ‘RuntimeError’, ‘RuntimeWarning’, ‘StopAsyncIteration’,
‘StopIteration’, ‘SyntaxError’, ‘SyntaxWarning’, ‘SystemError’, ‘SystemExit’, ‘TabError’,
‘TimeoutError’, ‘True’, ‘TypeError’, ‘UnboundLocalError’, ‘UnicodeDecodeError’,
‘UnicodeEncodeError’, ‘UnicodeError’, ‘UnicodeTranslateError’, ‘UnicodeWarning’, ‘UserWarning’,
‘ValueError’, ‘Warning’, ‘WindowsError’, ‘ZeroDivisionError’, ‘build_class’, ‘debug’, ‘doc’, ‘import’,
‘loader’, ‘name’, ‘package’, ‘spec’, ‘abs’, ‘all’, ‘any’, ‘ascii’, ‘bin’, ‘bool’, ‘breakpoint’, ‘bytearray’,
‘bytes’, ‘callable’, ‘chr’, ‘classmethod’, ‘compile’, ‘complex’, ‘copyright’, ‘credits’, ‘delattr’, ‘dict’, ‘dir’,
‘divmod’, ‘enumerate’, ‘eval’, ‘exec’, ‘exit’, ‘filter’, ‘float’, ‘format’, ‘frozenset’, ‘getattr’, ‘globals’,
‘hasattr’, ‘hash’, ‘help’, ‘hex’, ‘id’, ‘input’, ‘int’, ‘isinstance’, ‘issubclass’, ‘iter’, ‘len’, ‘license’, ‘list’,
‘locals’, ‘map’, ‘max’, ‘memoryview’, ‘min’, ‘next’, ‘object’, ‘oct’, ‘open’, ‘ord’, ‘pow’, ‘print’, ‘property’,
‘quit’, ‘range’, ‘repr’, ‘reversed’, ‘round’, ‘set’, ‘setattr’, ‘slice’, ‘sorted’, ‘staticmethod’, ‘str’, ‘sum’,
‘super’, ‘tuple’, ‘type’, ‘vars’, ‘zip’]

异常的继承关系：
BaseException 是所有异常的父类
断言测试

14.3 自定义异常

14.3.1 finally 关键字
finally:必须要执行的代码（释放资源、释放内存）

def dd(em):
    try:
        int(input())
        print('hello')
        return  'A'
    except Exception as e:
        print('出现异常')
        return 'B'
    else:
        print('没有出现异常')
    finally:
        print('无论如何都会打印')
        return 'C'

e=dd(input('>>>>>>>:'))
print(e)

def demo(msg):
	try:
		int(input(msg))
		print("hello world")
		return "A"
	except Exception as e:
		print("处理异常")
		return "B"
	finally:
		print("释放资源")
		return "C"
res = demo(input(">>>>>:"))
print(res)

注意：函数中，如果return后面存在finally,那么代码并不会直接返回，而是需要执行finally，再执行返回，所以finally会在return前执行

14.3.2 自定义异常
需要创建一个类，需要继承异常的父类（BaseException Exception）

#自定义异常
class myException(Exception):
    def __init__(self,m):   #重写
        Exception.__init__(self,m)

    def login(username,password):
        #raise 关键字  抛出异常
        if username==None or username.strip()=='':
            raise myException('对不起，用户名不能为空')
        if password==None or password.strip()=='':
            raise myException('对不起，密码不能为空')

if __name__ == '__main__':
    try:
        login(None,None)
    except Exception as e:
        print('异常是',e)

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