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

[Paul Rubin]

Paul Rubin  writes:
> Re your other post: yes, PEP 363 explains how the "with" statement

Whoops, 343. 
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Re: try...except...finally problem in Python 2.5

[Paul Rubin]

Steven D'Aprano <[EMAIL PROTECTED]> writes:
> 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.
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Re: try...except...finally problem in Python 2.5

[Steven D'Aprano]

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?


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

[Steven D'Aprano]

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

> Steven D'Aprano <[EMAIL PROTECTED]> writes:
>> self.isDataLoaded = False
>> try:
>> f = open(self.filename, 'rb')
>> f.seek(DATA_OFFSET)
>> self.__data = f.read(DATA_SIZE)
>> self.isDataLoaded = 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()?



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

[Paul Rubin]

Steven D'Aprano <[EMAIL PROTECTED]> writes:
> self.isDataLoaded = False
> try:
> f = open(self.filename, 'rb')
> f.seek(DATA_OFFSET)
> self.__data = f.read(DATA_SIZE)
> self.isDataLoaded = True
> except:
> pass
> else:
> pass
> 
> (apart from being four lines shorter)

Your version never closes the file.
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Re: try...except...finally problem in Python 2.5

[Steven D'Aprano]

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

> "redawgts" <[EMAIL PROTECTED]> writes:
>> try:
>> f = file(self.filename, '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.isDataLoaded = False
> with open(self.filename, 'rb') as f:
> f.seek(DATA_OFFSET)
> self.__data = f.read(DATA_SIZE)
> self.isDataLoaded = 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.isDataLoaded = False
try:
f = open(self.filename, 'rb')
f.seek(DATA_OFFSET)
self.__data = f.read(DATA_SIZE)
self.isDataLoaded = True
except:
pass
else:
pass

(apart from being four lines shorter)



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

[redawgts]

Thanks everybody, that helped alot.

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

[Paul Rubin]

"redawgts" <[EMAIL PROTECTED]> writes:
> try:
> f = file(self.filename, '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.isDataLoaded = False
with open(self.filename, 'rb') as f:
f.seek(DATA_OFFSET)
self.__data = f.read(DATA_SIZE)
self.isDataLoaded = True

and that should handle everything, closing the file automatically.
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Re: try...except...finally problem in Python 2.5

[Larry Bates]

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.filename, 'rb')
> f.seek(DATA_OFFSET)
> self.__data = f.read(DATA_SIZE)
> self.isDataLoaded = True
> except:
> self.isDataLoaded = 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.filename, 'rb')
f.seek(DATA_OFFSET)
self.__data = f.read(DATA_SIZE)
self.isDataLoaded = True

except:
isDataLoaded=False

else:
f.close()

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

[Andrew Koenig]

"redawgts" <[EMAIL PROTECTED]> wrote in message 
news:[EMAIL PROTECTED]

>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.filename, 'rb')
>f.seek(DATA_OFFSET)
>self.__data = f.read(DATA_SIZE)
>self.isDataLoaded = True
>except:
>self.isDataLoaded = 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.



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

[Robert Kern]

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.filename, 'rb')
> f.seek(DATA_OFFSET)
> self.__data = f.read(DATA_SIZE)
> self.isDataLoaded = True
> except:
> self.isDataLoaded = 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.filename, '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

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

[Jean-Paul Calderone]

On 14 Feb 2007 11:41:29 -0800, redawgts <[EMAIL PROTECTED]> 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.filename, 'rb')
>f.seek(DATA_OFFSET)
>self.__data = f.read(DATA_SIZE)
>self.isDataLoaded = True
>except:
>self.isDataLoaded = 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.
>

f is not bound until the first line of the try suite has completely
successfully executed.  If it fails to do this, for example because
self.filename is an attribute error, or the filename given by that
attribute does not exist, then the finally suite will execute before
f has been bound, and the UnboundLocalError will mask the real error.

Jean-Paul
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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.filename, 'rb')
f.seek(DATA_OFFSET)
self.__data = f.read(DATA_SIZE)
self.isDataLoaded = True
except:
self.isDataLoaded = 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.

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