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'
'''

 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)