Changing a line in a text file

[kah]

How do I change a line in a file??

For example I have the follwing text in my file:

line1
line2
line3
line4

How do I replace 'line2' with 'newline'. Since the write operation in
python will overwrite everything.

Regards,
Kah

 

[Michael Hoffman]

How do I change a line in a file??

For example I have the follwing text in my file:

line1
line2
line3
line4

How do I replace 'line2' with 'newline'. Since the write operation in
python will overwrite everything.

This is the best I can figure out what you mean:

lines = []
for line in file("myfile.txt"):
if line == "line2\n":
line = "newline\n"
lines.append(line)

file("myfile.txt", "W").writelines(lines)

 

[kah]

Hi,

the example provided by Vishnu is quite close to what I want but it
still required me to write all the data back to the file.

Is there any way where I can just write a particular line?

Regards,
Kah

 

[Steve Holden]

Hi,

the example provided by Vishnu is quite close to what I want but it
still required me to write all the data back to the file.

Is there any way where I can just write a particular line?

If you are asking whether you can update a file in place, the answer is
"yes" - look up "lseek" in the Python documentation (module os, under
"File Descriptor Operations".

However, you asked about replacing one line with another of a different
length: since this will mean changing the offsets of all subsequent
bytes you have no way to do this other than writing out the whole
content of the file following the modification. You would also have to
ensure that you truncated the file to the correct length.

In general, although they don't make it obvious that they are doing so
most programs that "change" files (text editors and the like) are really
writing new copies.

regards
Steve

 

[Miki Tebeka]

Hello kah,

How do I change a line in a file??

For example I have the follwing text in my file:

line1
line2
line3
line4

How do I replace 'line2' with 'newline'. Since the write operation in
python will overwrite everything.

See http://docs.python.org/lib/module-fileinput.html (inplace=1 is what you
want).

Bye.
--
------------------------------------------------------------------------
Miki Tebeka <[email protected]>
http://tebeka.bizhat.com
The only difference between children and adults is the price of the toys

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[Larry Bates]

You might be able to use "edge" case to make this simple.

1) If the line you are replacing is unique in the file

and

2) File can easily fit in memory

you can write:

fp=open(filename, 'r')
contents=fp.read()
fp.close()
newcontents=newline.join(contents.split(line2))
fp=open(filename, 'w')
fp.write(newcontents)
fp.close()

For small files this is very efficient.

Larry Bates

 

[Kirk Job Sluder]

kah wrote:
However, you asked about replacing one line with another of a
different length: since this will mean changing the offsets of all
subsequent bytes you have no way to do this other than writing out the
whole content of the file following the modification. You would also
have to ensure that you truncated the file to the correct length.

In general, although they don't make it obvious that they are doing so
most programs that "change" files (text editors and the like) are
really writing new copies.

In addition, I would argue that editing a file in place using a
non-interactive program is dangerous and bad practice in general. By
the time you find a bug in your edit script, the original is lost. This
is something I learned from bitter experience when I tried to be smart
and make script-based edits over entire directories of html files.

In unix shell scripting idiom, I would do something like:

mv file file.bak
sed -e 'g/oldline/c newline' < file.bak > file

And yes, I know that some versions of sed have the --in-place option.

Then, I would check for side effects:

diff file file.bak

All of this can be done in python, however I'm not overly familiar with
difflib and it seems to require both versions of the file in memory. So
an external diff might be better.

import os
os.rename(foo,foo.bak)
infile = open(foo.bak,'r')
outfile = open(foo,'w')
for line infile:
#test and substitution code block
outfile.write(line)

Using separate input and output files also has the advantage of being
memory efficient.

 

[Simon Brunning]

In addition, I would argue that editing a file in place using a
non-interactive program is dangerous and bad practice in general. By
the time you find a bug in your edit script, the original is lost. This
is something I learned from bitter experience when I tried to be smart
and make script-based edits over entire directories of html files.

See http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/56037