printing error message from an Exception

[Peter Otten]

mark jason wrote:

> hi
> I was trying out some file operations and was trying to open a non
> existing file as below
> 
> def do_work(filename):
>     try:
>         f = open(filename,"r");
>         print 'opened'
>     except IOError, e:
>         print 'failed',e.message
>     finally:
>         f.close()
>         print 'closed'
> 
> if __name__=='__main__':
>     do_work("C:\code\misc.txt")   # there is no such file
> 
> I am getting an error and a warning
> 
> DeprecationWarning: BaseException.message has been deprecated as of
> Python 2.6
>   print 'failed',e.message
> 
> UnboundLocalError: local variable 'f' referenced before assignment
> 
> Is there a way to overcome the DeprecationWarning? I wanted to print
> the error message from the IOError.How do I do this?
> Also ,what should I do about the UnboundLocalError?

You are mixing two things here:

(1) print an error message when the file cannot be opened
(2) ensure that the file will always be closed

If you attack both aspects separately they become simpler:

# 1
try:
   f = open(...)
except IOError as e:
    print "Failed", e

# 2a
f = open(...)
try:
  print "opened"  
finally:
   f.close()

which can be simplified to

# 2b
with open(...) as f:
   print "opened"

Putting 1 and 2b together:

try:
    with open(...) as f:
        print "opened"
except IOError as e:
    print "Failed", e

There is a disadvantage though: an IOError that is raised in the with-suite 
will also be caught. If you're concerned about that you can use:

try:
    f = open(...)
except IOError as e:
    print "Failed", e
else:
    with contextlib.closing(f):
        print "opened"