Question

<代码>le 和.finally关于例外处理章节是否多余? 例如,以下两部法典是否有什么区别?

try:
    foo = open("foo.txt")
except IOError:
    print("error")
else:
    print(foo.read())
finally:
    print("finished")

and

try:
    foo = open("foo.txt")
    print(foo.read())
except IOError:
    print("error")
print("finished")

More generally, can t the contents of else always be moved into the try, and can t the contents of finally just be moved outside the try/catch block? If so, what is the purpose of else and finally? Is it just to enhance readability?

Answer 1

finally is executed regardless of whether the statements in the try block fail or succeed. else is executed only if the statements in the try block don t raise an exception.

Answer 2

如果您将<代码>*>le栏内所载内容移至>>>>,则您还将列出在<代码>中可能出现的例外情况。如果是,


print(foo.read())

in your example throws an IOError, your first code snippet won t catch that error, while your second snippet will.  You try to keep try blocks as small as possible generally to really only catch the exceptions you want to catch.
The finally block gets always executed, no matter what.  If for example the try block contains a return statement, a finally block will still be executed, while any code beneath the whole try/except block won t.

Answer 3

No matter what happens, the block in the finally always gets executed. Even if an exception wasn t handled or the exception handlers themselves generate new exceptions.

Answer 4

try:
   print("I may raise an exception!")
except:
   print("I will be called only if exception occur!")
else:
   print("I will be called only if exception didn t occur!")
finally:
   print("I will be called always!")

Answer 5


 始终如一地实施
 如果没有例外。

我倾向于将该守则列入<代码>中,即最终栏,在<代码>try/code>后执行,但区块除外。

I prefer to put the code in else block which is executed if the try clause does not raise an exception same like this

Finally

try:
  f = open("file.txt")
  f.write("change file")
except:
  print("wrong")
finally:
  f.close()

Else

try:
   f = open("file.txt")
   f.write("change file")
except:
  print("wrong")
else:
  print("log => there is not any exception")
finally:
    f.close()

Answer 6

There are 3 possible "states": never occurred, handled and unhandled. You can map the control flow of the try-catch-else-finally clause into these 3 states like that:

from traceback import print_last

e_state =  unhandled exception 
try:
    # cause an exception here [or don t]
except SomeException as e: # use a suitable [or not] exception type here
    e_state =  handled exception 
    print( in "except" )
else:
    e_state =  no exception 
    print( in "else" )
finally:
    print(f in "finally". {e_state} occurred )
    if e_state ==  handled exception :
        print_last() # since the exception was caught - explicitly inform about it

Full examples below:

1. Handled Exception

from traceback import print_last

e_state =  unhandled exception 
try:
    1 / 0
except ZeroDivisionError as e:
    e_state =  handled exception 
    print( in "except" )
else:
    e_state =  no exception 
    print( in "else" )
finally:
    print(f in "finally". {e_state} occurred )
    if e_state ==  handled exception :
        print_last()

产出:

in "except"
in "finally". handled exception occurred

Traceback (most recent call last):
  File "...IPython/core/interactiveshell.py", line 3251, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File ".../T/ipykernel_59815/1316012763.py", line 5, in <module>
    1 / 0
ZeroDivisionError: division by zero

2. Unhandled Exception

from traceback import print_last

e_state =  unhandled exception 
try:
    1 / 0
except KeyError as e:
    e_state =  handled exception 
    print( in "except" )
else:
    e_state =  no exception 
    print( in "else" )
finally:
    print(f in "finally". {e_state} occurred )
    if e_state ==  handled exception :
        print_last()

产出:

in "finally". unhandled exception occurred

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Input In [14], in <module>
      3 e_state =  unhandled exception 
      4 try:
----> 5     1 / 0
      6 except KeyError as e:
      7     e_state =  handled exception 

ZeroDivisionError: division by zero

3. No Exception

from traceback import print_last

e_state =  unhandled exception 
try:
    1 / 2
except ZeroDivisionError as e:
    e_state =  handled exception 
    print( in "except" )
else:
    e_state =  no exception 
    print( in "else" )
finally:
    print(f in "finally". {e_state} occurred )
    if e_state ==  handled exception :
        print_last()

产出:

in "else"
in "finally". no exception occurred

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