Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Peter Otten <__peter__@web.de>
Newsgroups: comp.lang.python
Subject: Re: Python path and append
Date: Mon, 25 Apr 2016 23:38:10 +0200
Organization: None
Lines: 60
Message-ID: <mailman.94.1461620307.32212.python-list@python.org>
References: <smbdhb5kjje2oandna4vj5udku9gh1h9oa@4ax.com> <27nshbp40p1llr231dqm31p754tvurkb8i@4ax.com> <nflnc1$7a4$1@dont-email.me> <n5qshb5tmq4gk6nvqmad44lb523ouoiji5@4ax.com> <HE1PR07MB1356FDA50BB26CB85681F3E2F0620@HE1PR07MB1356.eurprd07.prod.outlook.com> <mailman.89.1461611345.32212.python-list@python.org> <02ushb9mntvtedeg5c7l33uhapt2j6nivu@4ax.com> <1461616112.2223232.589261113.1AB67247@webmail.messagingengine.com> <nfm2o7$j0h$1@ger.gmane.org>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
User-Agent: KNode/4.13.3
Precedence: list
Xref: csiph.com comp.lang.python:107631

Random832 wrote:

> On Mon, Apr 25, 2016, at 16:15, Seymore4Head wrote:
>> Thanks for the tip.
>> 
>> Still broke.  :(
>> 
>> f = open('wout.txt', 'r+')
>> for line in f:
>>     if line=="":
>>         exit
>>     line=line[:-1]
>>     line=line+" *"
>>     f.write(line)
>>     print line
>> f.close()
> 
> Your problem is that after you read the first line, your file "cursor"
> is positioned after the end of that line. So when you write the modified
> version of the line, it ends up after that. And then when you write it,
> the cursor is wherever the end of that is.
> 
> So if you start with this:
> AAA
> BBB
> CCC
> 
> You'll end up with this:
> AAA
> AAA* [this overwrites "BBB_C" with "AAA*_" if _ is the line break]
> CC
> CC*
> 
> There's no good way around this. You can either read the whole file into
> memory at once into a list, then rewind (look at the seek function) and
> write the lines out of the list, or you can write to a *different* file
> than the one you're reading.

You can leave the details to python though:

$ cat sample.txt
alpha
beta
gamma
$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import fileinput
>>> for line in fileinput.input("sample.txt", inplace=True):
...     print line.rstrip("\n"), "*"
... 
>>> 
$ cat sample.txt 
alpha *
beta *
gamma *

Too much magic for my taste, but the OP might like it.