python 笔记 异常处理

1 try-except

比如我们要打开一个不存在的文件,它会报错

f=open('no.txt','r')
f.read()

'''
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-54-2709fabe0da7> in <module>
----> 1 f=open('no.txt','r')
      2 f.read()

FileNotFoundError: [Errno 2] No such file or directory: 'no.txt'
'''

我们可以用try-except 实现

try:
    f=open('no.txt','r')
    print(f.read())
except FileNotFoundError as e:
    print(e)
    f=open('no.txt','w')
    f.write('new file')
    f.close()

'''
[Errno 2] No such file or directory: 'no.txt'
'''

python 笔记 异常处理

 但是如果我们except里面使用了不匹配的error 还是会报错

try:
    f=open('no1.txt','r')
    print(f.read())
except KeyError as e:
    print(e)
    f=open('no1.txt','w')
    f.write('new file')
    f.close()

'''
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-56-5e0e21b6da44> in <module>
      1 try:
----> 2     f=open('no1.txt','r')
      3     print(f.read())
      4 except KeyError as e:
      5     print(e)

FileNotFoundError: [Errno 2] No such file or directory: 'no1.txt'
'''

解决方法是使用基类Error

try:
    f=open('no1.txt','r')
    print(f.read())
except BaseException as e:
    print(e)
    f=open('no1.txt','w')
    f.write('new file')
    f.close()

2 多except异常处理

d = {"name": "f1", "age": 2}
l = [1,2,3]

try:
    v = d["gender"]
    l[3] = 4
except KeyError as e:
    print("key error for:", e)
    d["gender"] = "x"
except IndexError as e:
    print("index error for:", e)
    l.append(4)
print(d)
print(l)

'''
key error for: 'gender'
{'name': 'f1', 'age': 2, 'gender': 'x'}
[1, 2, 3]
'''

        但是此时有一点需要注意:它不会同时处理字典的 KeyError 和列表的 IndexError

        因为在程序顺序执行的时候,只要是报错了, 那么就会终止错误之后的代码,进入错误 回收 环节。这个回收环节在上面的案例中也就是 except 的错误处理环节。 

       换句话说,except语句只能执行一个。

        所以上述例子中,列表l并没有被append

3 try-except-else 语句

try-except和前面的一样,在 else 中处理没有报错的情况。

l = [1,2,3]
try:
    l[3] = 4
except IndexError as e:
    print(e)
else:
    print("no error, now in else")
    print(l)
'''
list assignment index out of range
'''
#此时是有异常的,所以执行的是except里面的语句

l = [1,2,3,-4]
try:
    l[3] = 4
except IndexError as e:
    print(e)
else:
    print("no error, now in else")
    print(l)
'''
no error, now in else
[1, 2, 3, 4]
'''
#此时没有异常,所以执行的是try和else里面的语句

4 try-except-finally 语句

try-except和前面的一样,finally中的语句,不管有没有异常,都会执行

在有些情况下,加不加finally,好像没啥区别

没有异常:

l = [1,2,3,-1]
try:
    l[3] = 4
except IndexError as e:
    print(e)
finally:
    print("reach finally")
'''
reach finally
'''
l = [1,2,3,-1]
try:
    l[3] = 4
except IndexError as e:
    print(e)

print("reach finally")
'''
reach finally
'''

有异常:

l = [1,2,3]
try:
    l[3] = 4
except IndexError as e:
    print(e)

print("reach finally")
'''
list assignment index out of range
reach finally
'''
l = [1,2,3]
try:
    l[3] = 4
except IndexError as e:
    print(e)
finally:
    print("reach finally")
'''
list assignment index out of range
reach finally
'''

所以,finally语句什么使用需要用到呢?

我们看下面这个例子:

l = [1,2,3]
try:
    l[3] = 4


print("reach finally")
'''
  File "<ipython-input-71-5a32d8788477>", line 6
    print("reach finally")
        ^
SyntaxError: invalid syntax
'''

l = [1,2,3]
try:
    l[3] = 4
finally:
    print("reach finally")
'''
reach finally
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-75-26ba6fe359fd> in <module>
      1 l = [1,2,3]
      2 try:
----> 3     l[3] = 4
      4 finally:
      5     print("reach finally")

IndexError: list assignment index out of range
'''

虽然都是报错,但是加了finally之后,就是先执行完finally中的语句,再报错

5 raise

手动触发异常

def no_negative(num):
    if num < 0:
        raise ValueError("I said no negative")
    return num

print(no_negative(-1))

'''
Traceback (most recent call last):
  File "/lib/python3.8/asyncio/futures.py", line 178, in result
    raise self._exception
  File "/lib/python3.8/asyncio/tasks.py", line 280, in __step
    result = coro.send(None)
  File "/lib/python3.8/site-packages/pyodide/_base.py", line 419, in eval_code_async
    return await CodeRunner(
  File "/lib/python3.8/site-packages/pyodide/_base.py", line 276, in run_async
    res = eval(last_expr, self.globals, self.locals)
  File "<exec>", line 6, in <module>
  File "<exec>", line 3, in no_negative
ValueError: I said no negative
'''

 注:这里raise 后面的error类型,必须是python 中定义的异常错误类型名称(见第6小节)

6 python 异常错误名称表

异常名称 描述
BaseException 所有异常的基类
SystemExit 解释器请求退出
KeyboardInterrupt 用户中断执行(通常是输入^C)
Exception 常规错误的基类
StopIteration 迭代器没有更多的值
GeneratorExit 生成器(generator)发生异常来通知退出
StandardError 所有的内建标准异常的基类
ArithmeticError 所有数值计算错误的基类
FloatingPointError 浮点计算错误
OverflowError 数值运算超出最大限制
ZeroDivisionError 除(或取模)零 (所有数据类型)
AssertionError 断言语句失败
AttributeError 对象没有这个属性
EOFError 没有内建输入,到达EOF 标记
EnvironmentError 操作系统错误的基类
IOError 输入/输出操作失败
OSError 操作系统错误
WindowsError 系统调用失败
ImportError 导入模块/对象失败
LookupError 无效数据查询的基类
IndexError 序列中没有此索引(index)
KeyError 映射中没有这个键
MemoryError 内存溢出错误(对于Python 解释器不是致命的)
NameError 未声明/初始化对象 (没有属性)
UnboundLocalError 访问未初始化的本地变量
ReferenceError 弱引用(Weak reference)试图访问已经垃圾回收了的对象
RuntimeError 一般的运行时错误
NotImplementedError 尚未实现的方法
SyntaxError Python 语法错误
IndentationError 缩进错误
TabError Tab 和空格混用
SystemError 一般的解释器系统错误
TypeError 对类型无效的操作
ValueError 传入无效的参数
UnicodeError Unicode 相关的错误
UnicodeDecodeError Unicode 解码时的错误
UnicodeEncodeError Unicode 编码时错误
UnicodeTranslateError Unicode 转换时错误
Warning 警告的基类
DeprecationWarning 关于被弃用的特征的警告
FutureWarning 关于构造将来语义会有改变的警告
OverflowWarning 旧的关于自动提升为长整型(long)的警告
PendingDeprecationWarning 关于特性将会被废弃的警告
RuntimeWarning 可疑的运行时行为(runtime behavior)的警告
SyntaxWarning 可疑的语法的警告
UserWarning 用户代码生成的警告

 参考内容:如何控制异常 try-except - 交互式学Python | 莫烦Python (mofanpy.com)

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