异常处理

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异常处理

定义 程序运行会出现各种各样的问题，例如语法编写错误或文件读写错误等，出现各种各样的bug（八阿哥） 默认情况下，程序会马上停止执行，并根据错误情况，反馈异常情况（显示栈追踪） 方便程序员进行排错和调试 哪个位置出了问题？ 哪种类型的错误？ 本质上讲，异常是一种类，并且有很多子类，基类为BaseException 举例

IOError
        异常说明
            输入输出异常
        异常代码
            file1 = open('file.txt','r')
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/IOError.py", line 1, in <module>
                file1 = open('file.txt','r')
            IOError: [Errno 2] No such file or directory: 'file.txt'
            Process finished with exit code 1
    ZeroDivisionError
        异常说明
            分母为0异常
        异常代码
            a = 1
            b = 0
            print a / b
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/ZeroDivisionError.py", line 3, in <module>
                print a / b
            ZeroDivisionError: integer division or modulo by zero
            Process finished with exit code 1
    TypeError
        异常说明
            类型异常，操作对本类型失效
        异常代码
            a = 1
            b = 'hello'
            print a + b
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/TypeError.py", line 3, in <module>
                print a + b
            TypeError: unsupported operand type(s) for +: 'int' and 'str'
            Process finished with exit code 1
    ValueError
        异常说明
            参数异常，无效的参数
        异常代码
            num = int('a')
            print num
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/ValueError.py", line 1, in <module>
                num = int('a')
            ValueError: invalid literal for int() with base 10: 'a'
            Process finished with exit code 1
    SyntaxError
        异常说明
            语法异常
        异常代码
            a = 1 +
        异常输出
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/SyntaxError.py", line 1
                a = 1 +
                      ^
            SyntaxError: invalid syntax
    NameError
        异常说明
            名字异常，对象未定义
        异常代码
            a = 1
            print a + b
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/NameError.py", line 2, in <module>
                print a + b
            NameError: name 'b' is not defined
            Process finished with exit code 1
    AttributeError
        异常说明
            属性异常，对象没有此属性/方法
        异常代码
            a = 1
            b = 'hello'
            b.coun()
            print b,a
        异常输出
            Traceback (most recent call last):
              File "/Users/jaykingchen/PycharmProjects/PINGINGLAB/6.Errors & Exceptions/ArritbuteError.py", line 3, in <module>
                b.coun()
            AttributeError: 'str' object has no attribute 'coun'
            Process finished with exit code 1

异常处理

功能 提高程序健壮性 实现程序的友好度 实现程序的功能调试

异常捕获

try...except...
        功能
            执行try语句，若没有异常，则正常执行，忽略except代码块
            若出现异常，则跳过try语句，执行except代码块
                try/except块的工作原理有点像if语句，if语句根据布尔表达式的结果决定如何做，而try/except块根据是否出现了异常决定如何做

举例 ​ 例1 ​ 捕获一个异常 ​ try: ​ a = 1 ​ b = 'hello' ​ print a + b ​ except TypeError: ​ try: ​ a = 1 ​ b = 'hello' ​ print a + b ​ except TypeError as e: ​ print 'except:', e ​ print 'invalid Type, please try again!' ​ 例2 ​ 捕获多个异常，同一处理结果 ​ try: ​ s = int(raw_input('how old are you:')) ​ print s ​ except (TypeError, ValueError): ​ print 'Please enter an integer value!' ​ 例3 ​ 捕获多个异常，不同处理结果 ​ try: ​ s = int(raw_input('how old are you:')) ​ print s ​ except TypeError : ​ >>> int(1,2) ​ Traceback (most recent call last): ​ File "<input>", line 1, in <module> ​ TypeError: int() can't convert non-string with explicit base ​ print 'Invalid type,Please enter an integer value!' ​ except ValueError: ​ how old are you:hi ​ Invalid value,Please enter an integer value! ​ print 'Invalid value,Please enter an integer value!' ​ 例4 ​ 捕获所有异常 ​ try: ​ a = 1 ​ b = 0 ​ print a / b ​ except : ​ print 'Error here, please try again!' ​ 例5 ​ 捕获异常，与while循环语句结合 ​ def get_age(): ​ while True: ​ try: ​ n = int(input('How old are you?')) ​ return n ​ except : ​ print('Please enter an integer value.') ​

            get_age()
        例6
            捕获异常，与else语句结合
                try:
                    a = 1
                    b = 2
                    print a
                except TypeError:
                    print 'invalid Type, please try again!'
                else:
                    若没有异常，则执行else语句
                    print b

异常清理

try...finally ​ 功能 ​ finally语句用于实现异常清理，不管最终try语句是否有异常，都必须执行finally语句 ​ 例如打开一个文件进行操作，不管是否异常，都必须在处理之后将其关闭，通过finally语句可以解决 ​ 举例 ​ try: ​ file1 = open('file.txt','w') ​ file1.read() ​ finally: ​ file1.close() ​ print 'close the file1' ​ try...except...finally ​ 功能 ​ finally语句用于实现异常清理，不管最终try语句是否有异常，都必须执行finally语句 ​ 例如打开一个文件进行操作，不管是否异常，都必须在处理之后将其关闭，通过finally语句可以解决 ​ 举例 ​ try: ​ file1 = open('file.txt','w') ​ file1.read() ​

        except IOError:
            print 'IOError, please try again!'
        
        finally:
            file1.close()
            print 'close the file1'
try...except...else...finally
    功能
        finally语句用于实现异常清理，不管最终try语句是否有异常，都必须执行finally语句
        例如打开一个文件进行操作，不管是否异常，都必须在处理之后将其关闭，通过finally语句可以解决
    举例
        try:
            file1 = open('file.txt','w+')
            file1.read()
        
        except IOError:
            print 'IOError, please try again!'
        
        else:
            print 'good!'
        
        finally:
            file1.close()
            print 'close the file1'
with...as
    功能
        with类似finally的功能，用于实现程序不管是否异常，都能正常关闭
            with 表达式 as 变量
    举例
            with open('file.txt','w') as f:
            不管是否出现异常，通过with语句执行的，最终需要清除
                f.read()

异常触发

raise ​ 功能 ​ Raise用于抛出异常，异常可以自己定义 ​ 语法 ​ raise [SomeException [, args [, traceback]]] ​ 第一个参数，SomeExcpetion是触发异常的名字 ​ 第二个符号为可选的 args(比如参数,值),来传给异常 ​ 最后一项参数,traceback,同样是可选的 ​ 举例 ​ 例1 ​ >>> raise NameError ​ Traceback (most recent call last): ​ File "<input>", line 1, in <module> ​ NameError ​ >>> raise TypeError ​ Traceback (most recent call last): ​ File "<input>", line 1, in <module> ​ TypeError ​ >>> raise IndexError ​ Traceback (most recent call last): ​ File "<input>", line 1, in <module> ​ IndexError ​ 例2 ​ def my_number(n): ​ if n > 0: ​ raise ValueError, 'invalid number! n = 0 or n < 0' ​ print 'n: ', n ​ my_number(1) ​ 例3 ​ def my_number(n): ​ if n > 0: ​ raise ValueError, 'invalid number! n = 0 or n < 0' ​ print 'n: ', n

try:
                my_number(1)
            except ValueError as e:
                print e, 'please try again!'
assert
    功能
        Assert断言是有条件地触发异常，如果不满足则抛出AssertionError异常
        相比raise，多了条件判断；但是不用像raise一样，抛出各种异常类型
    语法
        assert expression[, arguments]
    举例
        例1
            >>> assert 1 == 1
            >>> assert 1 == 2
            Traceback (most recent call last):
              File "<input>", line 1, in <module>
            AssertionError
            >>> assert 1 == 2, 'one does not equal two!'
            Traceback (most recent call last):
              File "<input>", line 1, in <module>
            AssertionError: one does not equal two!
        例2
            try:
                assert 1 == 2, 'one does not equal two!'
            except AssertionError:
                print 'please try again!'
        例3
            def my_number(n):
                assert n > 0, 'invalid number! n = 0 or n < 0'
                print 'n: ', n
            
            my_number(10)
            my_number(-1)​

来源：oschina

链接：https://my.oschina.net/u/4442211/blog/3212267