On 2014-01-30 13:02, Chris Angelico wrote:
On Thu, Jan 30, 2014 at 11:05 PM, Dave Angel <da...@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?
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