python try:except:finally

Question:

# Open new file to write
file = None
try:
    file = open(filePath, 'w')
except IOError:
    msg = ("Unable to create file on disk.")
    file.close()
    return
finally:
    file.write("Hello World!")
    file.close()

The above code is ripped from a function. One of the user’s system is reporting an error in line:

file.write("Hello World!")

error:

AttributeError: 'NoneType' object has no attribute 'write'

Question is, If python is failed to open given file, ‘except’ block executes and it has to
return, but control is getting transferred to the line that is throwing given error. The value of ‘file’ variable is ‘None’.

Any pointers?

Asked By: user354051

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Source

Answers:

You shouldn’t be writing to the file in the finally block as any exceptions raised there will not be caught by the except block.

The except block executes if there is an exception raised by the try block. The finally block always executes whatever happens.

Also, there shouldn’t be any need for initializing the file variable to none.

The use of return in the except block will not skip the finally block. By its very nature it cannot be skipped, that’s why you want to put your “clean-up” code in there (i.e. closing files).

So, if you want to use try:except:finally, you should be doing something like this:

try:
    f = open("file", "w")
    try:
        f.write('Hello World!')
    finally:
        f.close()
except IOError:
    print 'oops!'

A much cleaner way of doing this is using the with statement:

try:
    with open("output", "w") as outfile:
        outfile.write('Hello World')
except IOError:
    print 'oops!'
Answered By: Acorn

finally always gets called in the “end”, even if an exception ocurrs. You can use this to make sure open resources are closed (for instance, a DB connection, a file, etc).

I think you misunderstood the semantics.

Your logic should be in the “try”, you should deal with exceptions in the “except” block, and “finally” executes no matter how your method terminates, use it to clean up.

Answered By: pcalcao

except does not execute (because type is IOError) it’s the finally part that throws another error of type AttributeError because file = None.

Answered By: TigOldBitties

If the file is not opened, the line file = open(filePath, 'w') fails, so nothing gets assigned to file.

Then, the except clause runs, but nothing is in file, so file.close() fails.

The finally clause always runs, even if there was an exception. And since file is still None you get another exception.

You want an else clause instead of finally for things that only happen if there was no exception.

    try:
        file = open(filePath, 'w')
    except IOError:
        msg = "Unable to create file on disk."
        return
    else:
        file.write("Hello World!")
        file.close()

Why the else? The Python docs say:

The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.

In other words, this won’t catch an IOError from the write or close calls. Which is good, because then reason woudn’t have been “Unable to create file on disk.” – it would have been a different error, one that your code wasn’t prepared for. It’s a good idea not to try to handle such errors.

Answered By: Petr Viktorin

what is the logic in including the

file.write("Hello World!")

inside the finally clause?? i think it must be put in try clause itself.

try:
        file = open(filePath, 'w')
        file.write("Hello World!")
except IOError:
        print("Unable to create file on disk.")
finally:
        file.close()
Answered By: Sreenath Nannat

It is always advisable to write your logic or code that might throw an exception in the try block and use the finally block for closing the resources.

Answered By: Rohit Goyal

You can do something like this:

try:
    do_some_stuff()
finally:
    cleanup_stuff()
Answered By: user5578789

Here is the most direct solution to your problem. I use the idiom of checking for file_obj != None in the finally block.

By the way, you should be aware that file is a Python class name, so you should choose a different variable name.

file_obj = None
try:
    file_obj = open(filePath, 'w')
except IOError:
    msg = ("Unable to create file on disk.")
    file_obj.close()
    return
finally:
    if file_obj != None:
        file_obj.write("Hello World!")
        file_obj.close()
Answered By: stackoverflowuser2010