try...except...finally problem in Python 2.5

[redawgts]

I keep getting this error "local variable 'f' referenced before
assignment" in the finally block when I run the following code.

try:
f = file(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
self.isDataLoad ed = False
finally:
f.close()

Can someone tell me what's wrong with the code? Am I doing something
wrong? I'm somewhat new to python but this makes sense to me.

[Andrew Koenig]

Re: try...except... finally problem in Python 2.5

"redawgts" <redawgts@gmail .comwrote in message
news:1171482088 .993720.69620@l 53g2000cwa.goog legroups.com...

>I keep getting this error "local variable 'f' referenced before
assignment" in the finally block when I run the following code.
>
try:
f = file(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
self.isDataLoad ed = False
finally:
f.close()
>
Can someone tell me what's wrong with the code? Am I doing something
wrong? I'm somewhat new to python but this makes sense to me.

If the call to file raises an exception, then variable f won't have been
created.

Here's a simple example:

def f():
throw 42
try:
x = f()
except:
print x

Try it and see what happens. While you're at it, you might try to figure
out what you would like it to print.

[Robert Kern]

Re: try...except... finally problem in Python 2.5

redawgts wrote:

I keep getting this error "local variable 'f' referenced before
assignment" in the finally block when I run the following code.
>
try:
f = file(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
self.isDataLoad ed = False
finally:
f.close()
>
Can someone tell me what's wrong with the code? Am I doing something
wrong? I'm somewhat new to python but this makes sense to me.

Move the "f = file(self.filen ame, 'rb')" above the try:. If opening the file
happens to throw an exception, then the assignment will never happen and there
will be no 'f' to close.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[Larry Bates]

Re: try...except... finally problem in Python 2.5

redawgts wrote:

I keep getting this error "local variable 'f' referenced before
assignment" in the finally block when I run the following code.
>
try:
f = file(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
self.isDataLoad ed = False
finally:
f.close()
>
Can someone tell me what's wrong with the code? Am I doing something
wrong? I'm somewhat new to python but this makes sense to me.
>

finally: block is executed even if there is an exception in which case f
hasn't been defined. What you want is:

try:
f = file(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True

except:
isDataLoaded=Fa lse

else:
f.close()

-Larry

[Paul Rubin]

Re: try...except... finally problem in Python 2.5

"redawgts" <redawgts@gmail .comwrites:

try:
f = file(self.filen ame, 'rb') ...
Can someone tell me what's wrong with the code?

Various people have explained the error: if the file open attempt
fails, f is never assigned. Doing it the right way (i.e. handling the
potential exceptions separately) with try/except statements is messy,
so it's worth mentioning that 2.5 adds the new "with" statement to
clean this up. I'm not using 2.5 myself yet so maybe someone will
have to correct me, but I think you'd write:

from __future__ import with_statement

self.isDataLoad ed = False
with open(self.filen ame, 'rb') as f:
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True

and that should handle everything, closing the file automatically.

[redawgts]

Re: try...except... finally problem in Python 2.5

Thanks everybody, that helped alot.

[Steven D'Aprano]

Re: try...except... finally problem in Python 2.5

On Wed, 14 Feb 2007 12:09:34 -0800, Paul Rubin wrote:

"redawgts" <redawgts@gmail .comwrites:

> try:
> f = file(self.filen ame, 'rb') ...
>Can someone tell me what's wrong with the code?

>
Various people have explained the error: if the file open attempt
fails, f is never assigned. Doing it the right way (i.e. handling the
potential exceptions separately) with try/except statements is messy,
so it's worth mentioning that 2.5 adds the new "with" statement to
clean this up. I'm not using 2.5 myself yet so maybe someone will
have to correct me, but I think you'd write:
>
from __future__ import with_statement
>
self.isDataLoad ed = False
with open(self.filen ame, 'rb') as f:
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
>
and that should handle everything, closing the file automatically.

I don't have Python 2.5 here to experiment, but how is that different from
this?


self.isDataLoad ed = False
try:
f = open(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
pass
else:
pass

(apart from being four lines shorter)



--
Steven D'Aprano

[Paul Rubin]

Re: try...except... finally problem in Python 2.5

Steven D'Aprano <steve@REMOVEME .cybersource.co m.auwrites:

self.isDataLoad ed = False
try:
f = open(self.filen ame, 'rb')
f.seek(DATA_OFF SET)
self.__data = f.read(DATA_SIZ E)
self.isDataLoad ed = True
except:
pass
else:
pass
>
(apart from being four lines shorter)

Your version never closes the file.

[Steven D'Aprano]

Re: try...except... finally problem in Python 2.5

On Wed, 14 Feb 2007 18:03:19 -0800, Paul Rubin wrote:

Steven D'Aprano <steve@REMOVEME .cybersource.co m.auwrites:

>self.isDataLoa ded = False
>try:
> f = open(self.filen ame, 'rb')
> f.seek(DATA_OFF SET)
> self.__data = f.read(DATA_SIZ E)
> self.isDataLoad ed = True
>except:
> pass
>else:
> pass
>>
>(apart from being four lines shorter)

>
Your version never closes the file.

Yes it does. Eventually f goes out of scope and is closed automatically.

I don't see where the "with" version closes the file either. How does it
know that I want to call the f's close() method, rather than, say,
f.exit() or f.do_something_ else()?



--
Steven D'Aprano

[Steven D'Aprano]

Re: try...except... finally problem in Python 2.5

Replying to my own question...

On Thu, 15 Feb 2007 13:17:00 +1100, Steven D'Aprano wrote:

I don't see where the "with" version closes the file either. How does it
know that I want to call the f's close() method, rather than, say,
f.exit() or f.do_something_ else()?

Ah, I *think* I see... file objects in Python 2.5 are objects that know
how to work with the "with" statement; that is they obey the "context
management" protocol and have __enter__ and __exit__ methods that do the
right thing to make everything work correctly.

http://docs.python.org/whatsnew/pep-343.html

Have I got it right?


--
Steven D'Aprano

[Paul Rubin]

Re: try...except... finally problem in Python 2.5

Steven D'Aprano <steve@REMOVEME .cybersource.co m.auwrites:

Yes it does. Eventually f goes out of scope and is closed automatically.

Oh right, however you can't really predict when the closure occurs,
unless you're relying on current CPython artifacts.

Re your other post: yes, PEP 363 explains how the "with" statement
calls the __exit__ method.

[Paul Rubin]

Re: try...except... finally problem in Python 2.5

Paul Rubin <http://phr.cx@NOSPAM.i nvalidwrites:

Re your other post: yes, PEP 363 explains how the "with" statement

Whoops, 343.