File manipulation (try: except: finally:)

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February 19th, 2013, 02:32 PM

dkoleary

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File manipulation (try: except: finally:)

Hi;

Still trying to learn python. I'm playing around with files now. The books/tutorials that I've been reading suggest having a 'finally:' clause to the try/except to close the file. What I'm trying, though, doesn't seem clean to me:

Code:

#### imports:
import sys

file = '/etc/servces'
try:
   fo = open(file)
except:
   print 'no such file'
   sys.exit(1)
finally:
   fo.close()

print 'you made it this far!'

The 'file' parameter is intentionally incorrect. At this particular juncture, I'd like to just print 'no such file' and exit. Yet, as the docs suggest, the finally: clause gets executed even with the sys.exit.

Code:

$ ./tn.py
$ ./tn.py
no such file
Traceback (most recent call last):
  File "./tn.py", line 22, in <module>
    fo.close()
NameError: name 'fo' is not defined

Is there a right way to do this? Basically, I'm looking for something akin to the perl

Code:

finally:
   fo.close() if (defined(fo))  # to steal some code from perl

I'd like to learn the right way to do this from the start instead of having to unlearn things later.. been down that road a wee bit too many times

Thanks for any hints/tips/suggestion.

// a few minutes later:

Just read in the python cookbook that the open shouldn't be in the try statement.. so, the test of the open would be the attempt to read the file, then? Does that catch all errors (permission problems, file not there, etc?) The code in the cookbook says:

Code:

fo = open(file)
try:
   for line in fo:  # chto-nibud' slyuchaets'ya zdes'
finally:
   fo.close()

// and yet a few minutes more:

I found something that works; but it seems kind of kludgey ( I refer the reader to my earlier statement about learning the right way...)

Code:

#### imports:
import sys

file = '/etc/servces'
try:
   fo = open(file)
except IOError:
   print 'no such file'
   sys.exit(1)
   
try:
   lines = fo.readlines()
finally:
   fo.close()

print 'you made it this far!'

So, there're two try stanzas: one to check for errors on the file and the other to check on errors for reading the file. It seems like there should be a cleaner way of doing that.

Once again, thanks for any hints/tips/suggestions.

// and yet later still (last time, I promise)

I think I came up with *a* right answer. Not sure if it's *the* right answer:

Code:

#### imports:
import sys

file = sys.argv[1]
try:
   fo = open(file)
   try:
      lines = fo.readlines()
   except:
      print "Can't read file:", file
   finally:
      fo.close()
except:
   print "Can't open file:", file
   sys.exit(1)
   
print 'you made it this far!'

execution:

Code:

$ ./tn.py /etc/services
you made it this far!
$ ./tn.py /etc/servces 
Can't open file: /etc/servces

That seems to satisfy the desire. Is that the accepted way to go about error checking file manipulation?

Thanks again.

Doug O'Leary

February 19th, 2013, 03:16 PM

Scorpions4ever

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Just do it something like this:

Code:

#!/usr/bin/python

import sys

file = '/etc/does_not_exist'

fo = None
try:
    fo = open(file)
except:
    print('No such file')
    sys.exit(0)
finally:
    if fo != None: fo.close()

print('Made it this far')

Note that I initialize fo (and set it to None) outside the try block so that it will be defined by the time it hits finally. Within the finally clause, I check to see if it is None or not and only close it if it is not None. This is similar to how the perl code does a:

Code:

close($fo) if defined($fo);

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February 19th, 2013, 03:16 PM

SuperOscar

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Quote:

Originally Posted by dkoleary

The books/tutorials that I've been reading suggest having a 'finally:' clause to the try/except to close the file.

I think that’s old information. The new correct way is to use “with”:

Code:

with open(file) as fo:
    # ...

I would check the existence of the file beforehand with os.path.isfile() but I guess wrapping the above in a try...except block is OK too.

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February 19th, 2013, 03:54 PM

dkoleary

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Join Date: Feb 2013

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Hey;

Thank you both! Very helpful. With some reading on the 'with' keyword, I'm down to:

Code:

#### imports:
import sys

file = sys.argv[1]

try:
   with open(file) as fo:
      lines = fo.readlines()
except IOError as e:
   print "Can't open file:", e
   sys.exit(1)
   
print 'you made it this far!'

The idea of pre-initializing the variable is quite clever. Wish I'd thought of that myself!

Thanks again.

Doug O'Leary

File manipulation (try: except: finally:)

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