try...except...finally problem in Python 2.5
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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 -- http://mail.python.org/mailman/listinfo/python-list
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.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 -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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. -- http://mail.python.org/mailman/listinfo/python-list
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.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 -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
Thanks everybody, that helped alot. -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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) -- Steven D'Aprano -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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()? -- Steven D'Aprano -- http://mail.python.org/mailman/listinfo/python-list
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 -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
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. -- http://mail.python.org/mailman/listinfo/python-list
Re: try...except...finally problem in Python 2.5
Paul Rubin http://[EMAIL PROTECTED] writes: Re your other post: yes, PEP 363 explains how the with statement Whoops, 343. -- http://mail.python.org/mailman/listinfo/python-list
