[Python知识库]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'
'''

?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

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