How to use writelines to append new lines to an existing file

[Nico Grubert]

Hi there,

I would like to open an existing file that contains some lines of text
in order to append a new line at the end of the content.

My first try was:
[color=blue][color=green][color=darkred]
>>> f = open('/tmp/myfile', 'w') #create new file for writing
>>> f.writelines('1 23') #write first line
>>> f.close()
>>> f = open('/tmp/myfile', 'w') #open existing file to append new line
>>> f.writelines('4 56')
>>> f.close()
>>> f = open('/tmp/myfile', 'r') # open file for reading
>>> f.read()[/color][/color][/color]
'456'

I supposed to have:[color=blue][color=green][color=darkred]
>>> f.read()[/color][/color][/color]
'123\n456\n'


Does f = open('/tmp/myfile', 'w') overwrite the existing file or does
f.writelines('4 56') replace the first line in the existing file?

Nico

[Grigoris Tsolakidis]

Re: How to use writelines to append new lines to an existing file

Use
file = open(open('/tmp/myfile', 'a')) the second time
when you want to append line

"Nico Grubert" <nicogrubert@gm ail.com> wrote in message
news:mailman.78 1.1127393534.50 9.python-list@python.org ...[color=blue]
> Hi there,
>
> I would like to open an existing file that contains some lines of text in
> order to append a new line at the end of the content.
>
> My first try was:
>[color=green][color=darkred]
> >>> f = open('/tmp/myfile', 'w') #create new file for writing
> >>> f.writelines('1 23') #write first line
> >>> f.close()
> >>> f = open('/tmp/myfile', 'w') #open existing file to append new line
> >>> f.writelines('4 56')
> >>> f.close()
> >>> f = open('/tmp/myfile', 'r') # open file for reading
> >>> f.read()[/color][/color]
> '456'
>
> I supposed to have:[color=green][color=darkred]
> >>> f.read()[/color][/color]
> '123\n456\n'
>
>
> Does f = open('/tmp/myfile', 'w') overwrite the existing file or does
> f.writelines('4 56') replace the first line in the existing file?
>
> Nico[/color]

[Max Erickson]

Re: How to use writelines to append new lines to an existing file

Nico Grubert <nicogrubert@gm ail.com> wrote in
news:mailman.78 1.1127393534.50 9.python-list@python.org :
[color=blue]
> Hi there,
>
> I would like to open an existing file that contains some lines of
> text in order to append a new line at the end of the content.
>
> My first try was:
>[color=green][color=darkred]
> >>> f = open('/tmp/myfile', 'w') #create new file for writing
> >>> f.writelines('1 23') #write first line
> >>> f.close()
> >>> f = open('/tmp/myfile', 'w') #open existing file to append
> >>> new line f.writelines('4 56')
> >>> f.close()
> >>> f = open('/tmp/myfile', 'r') # open file for reading
> >>> f.read()[/color][/color]
> '456'
>
> I supposed to have:[color=green][color=darkred]
> >>> f.read()[/color][/color]
> '123\n456\n'
>
>
> Does f = open('/tmp/myfile', 'w') overwrite the existing file
> or does f.writelines('4 56') replace the first line in the
> existing file?
>
> Nico[/color]

There is a good explanation in the tutorial:

http://docs.python.org/tut/node9.html#SECTION009200000000000000000

and more detail is available in the docs for builtin functions:

http://docs.python.org/lib/built-in-funcs.html

(look under file() but probably still use open())

That said, open(file, 'a') will open an existing file to append.

max