Try-except-finally paradox

[MRAB]

On 2014-01-30 13:02, Chris Angelico wrote:
> On Thu, Jan 30, 2014 at 11:05 PM, Dave Angel <davea at davea.name> wrote:
>> The finally has to happen before any return inside the try or the
>>  except.  And once you're in the finally clause you'll finish it
>>  before resuming the except clause.  Since it has a return,  that
>>  will happen before the other returns. The one in the except block
>>  will never get reached.
>>
>> It's the only reasonable behavior., to my mind.
>
> It's arguable that putting a return inside a finally is unreasonable
> behaviour, but that's up to the programmer. A finally clause can be
> used to do what might be done in C++ with a destructor: "no matter how
> this function/block exits, do this as you unwind the stack". In C++, I
> might open a file like this:
>
> void func()
> {
>      ofstream output("output.txt");
>      // do a whole lot of stuff ...
>      // at the close brace, output.~output() will be called, which will
> close the file
> }
>
> In Python, the equivalent would be:
>
> def func():
>      try:
>          output = open("output.txt", "w")
>          # do a whole lot of stuff ...
>      finally:
>          output.close()
>
> (Actually, the Python equivalent would be to use a 'with' clause for
> brevity, but 'with' uses try/finally under the covers, so it comes to
> the same thing.) The concept of the finally clause is: "Whether
> execution runs off the end, hits a return statement, or throws an
> exception, I need you do this before anything else happens". Having a
> return statement inside 'finally' as well as in 'try' is a bit of a
> corner case, since you're now saying "Before you finish this function
> and return something, I need you to return something else", which
> doesn't usually make sense. If you think Python's behaviour is
> confusing, first figure out what you would expect to happen in this
> situation :)
>
One of the reasons that the 'with' statement was added was to prevent
the mistake that you've just done. ;-)

What if the file can't be opened?