On 2018-10-11 11:44, Iranna Mathapati wrote:
> Hi Team,
>
> How to replace particular line text with new text on a file
> i have below code but its writing whole code.
>
> def replace_line(file_name, line_num, text):
> lines = open(file_name, 'r').readlines()
> lines[line_num] = text
> out = open(file_name, 'w')
> out.writelines(lines) <<<<< *writing back whole file instead of
> particular line*
> out.close()
> replace_line('stats.txt', 0, 'good')
>
>
> Thanks,
> Iranna M
>
Can't be done.
There's no easy way to write to the middle of a file (though I think it
can be done by mmap'ing a file) -- and even if you manage to do that,
you'd have to write back anything after the changed line if the length
of that line changed by as little as a byte.
If you really want, you can avoid writing back anything *before* your
change by carefully using truncate():
def replace_line(file_name, line_num, new_text):
# Use a with statement to make sure the file is closed
with open(file_name, 'r+') as f:
# Ignore the first (line_num) lines
for _ in range(line_num):
f.readline()
# Save position, skip the line to be replaced
line_start = f.tell()
f.readline()
# We're keeping the rest of the file:
remaining_content = f.read()
# Chop off the file's tail!
f.seek(line_start)
f.truncate()
# Write the new tail.
if not new_text.endswith('\n'):
new_text = new_text + '\n'
f.write(new_text)
f.write(remaining_content)
That works, but your solution is more elegant.* This solution might make
sense for fairly large files where you can guarantee that the change
will be near the end.
* though you should close the file after you read it, preferable using a
with statement.
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