Can I change one line in a file without rewriting the whole thing?

[J. J. Ramsey]

In Perl, there is a module called "Tie::File". What it does is tie a
list to each line of a file. Change the list, and the file is
automatically changed, and on top of this, only the bits of the file
that need to be changed are written to disk. At least, that's the
general idea.

I was wondering if something roughly similar could be done in Python,
or at the very least, if I can avoid doing what amounts to reading the
whole file into memory, changing the copy in memory, and writing it
all out again.

[Gabriel Genellina]

En Fri, 13 Jul 2007 23:46:24 -0300, J. J. Ramsey <jj******@pobox.com>
escribió:

In Perl, there is a module called "Tie::File". What it does is tie a
list to each line of a file. Change the list, and the file is
automatically changed, and on top of this, only the bits of the file
that need to be changed are written to disk. At least, that's the
general idea.

That usually means, rewriting from the first modified line to the end of
the file.

I was wondering if something roughly similar could be done in Python,
or at the very least, if I can avoid doing what amounts to reading the
whole file into memory, changing the copy in memory, and writing it
all out again.

Simplest aproach:

lines = list(open("myfile.txt"))
del lines[13]
lines[42] = "Look ma! Replacing line 42!\n"
open("myfile.txt","w").writelines(lines)

This of course reads the whole file in memory, but it's a compact way if
you require random line access.
If you can serialize the file operations, try using the fileinput module
with inplace=1.

(Having a true Tie::File implementation for Python would be a nice
addition to the available tools...)

--
Gabriel Genellina