[Tutor] Trying to extract the last line of a text file

[János Juhász]

> Danny Yoo wrote:
> >
> >>>>> file('filename.txt').readlines()[-1]
> >>>> Not to hijack the thread, but what stops you from just putting a
> >>>> file.close() after your example line?
> >>> Which file should file.close() close?  The problem is that we don't
> >>> have a handle on the particular file we want to close off.
> >>>
> >> Oh wow.. I totally missed that... nevermind.. ignore that question =D
> >
> >
> > Hi Chris,
> >

> Wow, that seems like overkill when you can just write
> f = open('filename.txt')
> f.readlines()
> f.close()

> In CPython (the regular Python that we usually talk about here,
> implemented in C) a file will be closed automatically as soon as there
> are no references to the file because CPython garbage collects objects
> immediately. This behaviour is not guaranteed by the language though and
> it is different in Jython.
> >
> >
> > This is similar in spirit to the idea of "autorelease" memory pools 
used
> > by the Objective C language.  We use some resource "manager" that does
> > keep a handle on resources.  That manager then has the power and
> > responsiblity to call close() at some point.  So one might imagine 
doing
> > something like:
> >

> In Python 2.5 you can use with: to do this:
> with open('filename.txt') as f:
> f.readlines()
> 
> f is guaranteed to be closed when the block exits.
> Kent

I made a small test about, what could happen with a file object,
that was opened for read, but left without closing.

# we need to put something into the test file
>>> fw1 = open(r'test.txt', 'w')
>>> fw1.write('written by fw1')
>>> fw1.close()
# so we have the test file

# we can open and read from it
>>> fr1 = open(r'test.txt', 'r')
>>> fr1.readlines()
['written by fw1']
# I left it opened.

# Another instance could be opened for read again
>>> fr2 = open(r'test.txt', 'r')
>>> fr2.readlines()
['written by fw1']
# I left the second instance opened eighter

# Someone rewrite the content of the file
>>> fw2 = open(r'test.txt', 'w')
>>> fw2.write('written by fw2')
>>> fw2.close()
# I closed it correctly after writing

# The instance opened for reading could be reread
>>> fr1.seek(0)
>>> fr1.readlines()
['written by fw2']


>>> fr2.readlines()
[]

# I have extended it a little
>>> fw2 = open(r'test.txt', 'w')
>>> fw2.write('written by fw2 but it is extended later')
>>> fw2.close()


>>> fr2.read()
' but it is extended later'
>>> 

I feel that, we can left opened files with open(filename).readlines() in a 
number of times,
but would be problematic to do it 100 000 times in the same script.


Yours sincerely, 
______________________________
Janos Juhasz