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

[Chris Hengge]

I thought my solution was the easiest.. but I guess everyone skipped it =P

On 10/19/06, Kent Johnson <kent37 at tds.net> wrote:
>
> 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,
> >
> > No, no, it's an interesting one.  It turns out that there IS a way to
> > sorta do what you're thinking:
> >
> > ############################################################
> > class FilePool:
> >      """A small demo class to show how we might keep track of
> >      files opened with us."""
> >      def __init__(self):
> >          self.pool = []
> >
> >      def open(self, filename):
> >          f = open(filename)
> >          self.pool.append(f)
> >          return f
> >
> >      def closeAll(self):
> >          for f in self.pool:
> >              f.close()
> >          self.pool = []
> > fp = FilePool()
> > ############################################################
> >
> >
> > Once we have FilePool, we might say something like:
> >
> > ##################################################################
> > print "the last line is:", fp.open('filename.txt').readlines()[-1]
> > fp.closeAll()
> > ##################################################################
>
> 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:
> >
> > ############################################
> > create a manager
> > try:
> >      ... # use resources that the manager doles out
> > finally:
> >      use the manager to close everything down
> > ############################################
>
> 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
>
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