Thanks all.
I think my fundamental error was in thinking of *menu.tx*t as the file and *
inp* as containing it's contents in some way. Much more helpful to think of
*inp* as the file and then do things to it.
Thanks again
Neil
This email is confidential and intended for addressee only .
On Mon, Jun 21, 2010 at 1:37 AM, Alex Hall <mehg...@gmail.com> wrote:
> On 6/20/10, Steven D'Aprano <st...@pearwood.info> wrote:
> > On Mon, 21 Jun 2010 09:02:55 am Alex Hall wrote:
> >> On 6/20/10, Neil Thorman <neil.thor...@gmail.com> wrote:
> > [...]
> >> >>>>inp = file("menu.txt", "r")
> >> >
> >> > *What is inp? What does it now contain?*
> >>
> >> It is now a reference to the location of the txt file.
> >
> > [pedantic]
> > No, it's actually an abstract data structure that wraps the actual file
> > itself. A reference to the *location* of the file would be:
> >
> > inp = "menu.txt"
> >
> > Locations are strings. File objects are file objects.
> > [/pedantic]
> >
> > But in a practical sense, pay no attention to the man behind the curtain
> > (the implementation details). inp is a file in every way which matters,
> > just like after:
> >
> > n = 42
> > x = 1.234
> >
> > n is an int and x a float in every way which matters.
> >
> >
> >> Python calls
> >> these file "objects", where an object is just something on which you
> >> can call functions. If you had a dog object you might call a "bark"
> >> method; here, we have a file object, so we can see what the file is.
> >> inp is not the file itself, just as any object is not the full
> >> object's info but rather a pointer to where that info is. If you were
> >> to print inp, I think you would get a memory address.
> >
> > You would get a string printed to the console, like printing *anything*.
> >
> >>>> print inp
> > <open file 'foo', mode 'w' at 0xb7d3a250>
> Oh right, the object's toString method (or whatever Python calls this;
> Java and Javascript call it toString, and it exists for all objects).
> >
> > That string happens to contain a hex number which looks like it could be
> > a memory address, but that's an implementation detail because CPython
> > doesn't sufficiently abstract its objects from the underlying C
> > implementation.
> >
> > Python file objects aren't "files" only in the sense that they exist in
> > memory rather than on disk in the file system, but other than that, I
> > believe your explanation is at too low a level to be helpful. Neil said
> > he's a beginner who hasn't done any programming since BASIC on an
> > Acorn, and you're talking about low-level details like memory
> > locations. Let me guess, you're also a C programmer?
> Good point, and sorry for going into too much detail, much of which is
> not on the mark anyway. :) No, I have hardly touched c++, but I am
> starting my final year of a computer science degree in a few months so
> I have had all the details of objects and how the computer actually
> accesses them in several classes.
> >
> > As far as coding in Python is concerned, inp = file(pathname) creates a
> > file object, which *is* a file in all practical sense. Everything else
> > is just an implementation detail, which could change depending on the
> > version of Python, the operating system, and the underlying hardware.
> Very true.
> >
> >
> > [...]
> >> >>>>print inp.readlines()
> >> >
> >> > ['spam & eggs\n', 'spam & chips\n', 'spam & spam']
> >> >
> >> > but if I do it again I get:
> >> >>>> print inp.readlines()
> >> >
> >> > []
> >> >
> >> > I'm baffled, why is inp now empty?
> >>
> >> I suspect you have hit the end of the file.
> >
> > Yes. readlines reads from the current file position, like all read
> > operations. To read all the text again, you have to reset the file
> > position with seek:
> >
> > inp.seek(0)
> >
> >
> >
> > --
> > Steven D'Aprano
> > _______________________________________________
> > Tutor maillist - Tutor@python.org
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> >
>
>
> --
> Have a great day,
> Alex (msg sent from GMail website)
> mehg...@gmail.com; http://www.facebook.com/mehgcap
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