[Tutor] Deleting Lines of text

[Danny Yoo]

On Tue, 16 Apr 2002 pythonhack@yahoo.com wrote:

> as far as i know, it's not possible to edit a file in place.  you'll

It's not too hard to do in-place replacements of characters in a file:

###
>>> f = open("test.txt", "w")
>>> f.write("hello world!\n")
>>> f.close()
>>> print open("test.txt").read()
hello world!

>>> f = open("test.txt", "r+")
>>> f.seek(1)
>>> f.write("a")
>>> f.seek(0)
>>> print f.read()
hallo world!

###

So in-place modification isn't bad: it's the insertions and deletions that
cause problems.  Insertions and deletions involve shifing all the
characters in a file either left or right when we do an insertion or
deletion.  Removing or deleting a line in-place is also problematic
because not all lines are the same length --- some lines are longer than
others.


Text editors provide the illusion that it's easy to edit a file in place.
What often happens is that the file is temporarily held in some data
structure like a list (or linked list!  *grin*0, and all manipulations are
done on this structure in memory. Lists are much easier to deal with than
files: it's relatively easy to do insertions or other things.

Finally, when the work is done, a "Save" command will write the contents
of the data structure back to disk.  That's why word processors make a
distinction between "editing" and "saving", because editing does
manipulation on a copy of the document in memory, rather than directly on
the file.