try, except, finally ?

[microphone_head]

Hi all,

I've been trying to tighten up my code by adding error handling using the try command, but I can't seem to get my head around the "finally" statement. I admit I have just been using Python with pygame to make games but I can't seem to find an occassion where I need to use the "finally" statement in a "try" block. I can only presume that I don't really understand it.

Does anyone have any scenarios where its proven more convenient to include the "finally" statement block?

"""
A module to try and work out the 'finally'
statement in a 'try' block.
"""

try:
    f = open("demofile.txt")
    f.write("Lorum Ipsum")
except:
    print("Something went wrong when writing to the file.")
finally:
    f.close()

PS. The above code requires the text file to be read-only.

Looking at the common example of cleaning up and not leaving a file open, I don't understand why I just don't just have the close statement without the "finally" block? Was it just introduced to act as a sort of self documentation?

[ichabod801]

why I just don't just have the close statement without the "finally" block?

You have an empty except statement. Generally you should be catching specific errors, like an IOError. If you got a different kind of error, the close call wouldn't execute without the finally block. Also, if you raise an error in your except block (which I do a fair amount to clarify error messages), it wouldn't execute without the finally block.

[Yoriz]

Here is an example

try:
    print('file open')
    raise PermissionError
except FileNotFoundError as exception:
    print(exception)
finally:
    print('file closed in finally')
    
print('file closed outside try/except/finally')

Error:
file open
file closed in finally
Traceback (most recent call last):
  File "C:\Users\Dave\Documents\Eclipse Workspace\Test\forum\forum_post.py", line 4, in <module>
    raise PermissionError
PermissionError

The code is expecting to get a FileNotFoundError but there is actually a unexpected PermissionError
The file close in the finally still happens, if there was no finally the one after would not close the file.

[microphone_head]

Wow ! I've read a few examples but I think I finally get it , thanks. That a great help.