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comp.lang.python

Re: Delete lines containing a specific word

Francesco Pietra

1/6/2008 9:34:00 PM

Steven:
Thanks. See below please (of very marginal interest)

--- Steven D'Aprano <steven@REMOVE.THIS.cybersource.com.au> wrote:

> On Sun, 06 Jan 2008 09:21:33 -0800, Francesco Pietra wrote:
>
> > Please, how to adapt the following script (to delete blank lines) to
> > delete lines containing a specific word, or words?
>
> That's tricky, because deleting lines from a file isn't a simple
> operation. No operating system I know of (Windows, Linux, OS X) has a
> "delete line" function.

As I am at Debian Linux, I do that with grep -v


>
> Do you really need to delete the lines in place? It would be much simpler
> to leave the original data as-is, and create a new file with just the
> lines that aren't deleted.
>
>
> > f=open("output.pdb", "r")
> > for line in f:
> > line=line.rstrip()
> > if line:
> > print line
> > f.close()
>
> How to adapt this script:
>
> First, think about what this script does. That is, it goes through each
> line, and if the line is not blank, it prints it.
>
> What do you want it to do instead? You want it to print the line if the
> line doesn't contain a specific word. So that's the first thing you need
> to change.
>
> Secondly, you might want the script to write its output to a file,
> instead of printing. So, instead of the line "print line", you want it to
> write to a file.

may be cumbersome, though I use 2>&1 | tee output file.pdb so that I can see
what happens on the screen and have the modified file.

>
> Before you can write to a file, you need to open it. So you will need to
> open another file: you will have two files open, one for input and one
> for output. And you will need to close them both when you are finished.
>
> Does that help you to adapt the script?
>
>
> > If python in Linux accepts lines beginning with # as comment lines,
> > please also a script to comment lines containing a specific word, or
> > words, and back, to remove #.
>
> The same process applies. Instead of "delete line", you want to "comment
> line".
>
>
>
> --
> Steven
>
>
> --
> http://mail.python.org/mailman/listinfo/p...
>



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6 Answers

Steven D'Aprano

1/6/2008 9:57:00 PM

0

On Sun, 06 Jan 2008 13:33:52 -0800, Francesco Pietra wrote:

> Steven:
> Thanks. See below please (of very marginal interest)
>
> --- Steven D'Aprano <steven@REMOVE.THIS.cybersource.com.au> wrote:
>
>> On Sun, 06 Jan 2008 09:21:33 -0800, Francesco Pietra wrote:
>>
>> > Please, how to adapt the following script (to delete blank lines) to
>> > delete lines containing a specific word, or words?
>>
>> That's tricky, because deleting lines from a file isn't a simple
>> operation. No operating system I know of (Windows, Linux, OS X) has a
>> "delete line" function.
>
> As I am at Debian Linux, I do that with grep -v

grep doesn't delete lines. grep matches lines. If you want to delete
them, you still have to do the rest of the job yourself.


>> Secondly, you might want the script to write its output to a file,
>> instead of printing. So, instead of the line "print line", you want it
>> to write to a file.
>
> may be cumbersome, though I use 2>&1 | tee output file.pdb so that I
> can see what happens on the screen and have the modified file.

Yes, matching lines and sending them to stdout is a better solution than
trying to delete them from a file.



--
Steven

Bjoern Schliessmann

1/6/2008 11:34:00 PM

0

Steven D'Aprano wrote:

> grep doesn't delete lines. grep matches lines. If you want to
> delete them, you still have to do the rest of the job yourself.

In which way does "grep -v mypattern myfile > myfile" not delete the
lines matching mypattern?

Regards,


Björn

--
BOFH excuse #184:

loop found in loop in redundant loopback

bukzor

1/6/2008 11:43:00 PM

0

On Jan 6, 3:33 pm, Bjoern Schliessmann <usenet-
mail-0306.20.chr0n...@spamgourmet.com> wrote:
> Steven D'Aprano wrote:
> > grep doesn't delete lines. grep matches lines. If you want to
> > delete them, you still have to do the rest of the job yourself.
>
> In which way does "grep -v mypattern myfile > myfile" not delete the
> lines matching mypattern?
>
> Regards,
>
> Björn
>
> --
> BOFH excuse #184:
>
> loop found in loop in redundant loopback

If you do that, you'll find an empty file at the end.

Grant Edwards

1/7/2008 12:42:00 AM

0

On 2008-01-06, Steven D'Aprano <steven@REMOVE.THIS.cybersource.com.au> wrote:
> On Sun, 06 Jan 2008 13:33:52 -0800, Francesco Pietra wrote:
>
>> Steven:
>> Thanks. See below please (of very marginal interest)
>>
>> --- Steven D'Aprano <steven@REMOVE.THIS.cybersource.com.au> wrote:
>>
>>> On Sun, 06 Jan 2008 09:21:33 -0800, Francesco Pietra wrote:
>>>
>>> > Please, how to adapt the following script (to delete blank lines) to
>>> > delete lines containing a specific word, or words?
>>>
>>> That's tricky, because deleting lines from a file isn't a simple
>>> operation. No operating system I know of (Windows, Linux, OS X) has a
>>> "delete line" function.
>>
>> As I am at Debian Linux, I do that with grep -v
>
> grep doesn't delete lines. grep matches lines.

grep does far more than that.

> If you want to delete them, you still have to do the rest of
> the job yourself.

Nonsense.

How is this not doing what the OP asks?

grep -v pattern infile >outfile; mv outfile infile

If you don't like explicitly using a second file, you can use
sed:

sed -i '/pattern/d' filename

>>> Secondly, you might want the script to write its output to a file,
>>> instead of printing. So, instead of the line "print line", you want it
>>> to write to a file.
>>
>> may be cumbersome, though I use 2>&1 | tee output file.pdb so that I
>> can see what happens on the screen and have the modified file.
>
> Yes, matching lines and sending them to stdout is a better
> solution than trying to delete them from a file.

If you're matching all lines that don't contain the pattern in
question, then matching all lines and sending them to stdout
_is_ a way to delete them.

--
Grant Edwards grante Yow! And furthermore,
at my bowling average is
visi.com unimpeachable!!!

Steven D'Aprano

1/7/2008 3:42:00 AM

0

On Mon, 07 Jan 2008 00:33:36 +0100, Bjoern Schliessmann wrote:

> Steven D'Aprano wrote:
>
>> grep doesn't delete lines. grep matches lines. If you want to delete
>> them, you still have to do the rest of the job yourself.
>
> In which way does "grep -v mypattern myfile > myfile" not delete the
> lines matching mypattern?

Okay, that will delete the lines matching mypattern. Unfortunately it
will also delete all the lines NOT matching mypattern as well. Try it for
yourself -- just not on anything you care about.

This is what happens when abstractions leak. You *think* you're deleting
lines, but you're not. That's just an abstraction, and when it leaks, you
break things.



--
Steven

Steven D'Aprano

1/7/2008 3:58:00 AM

0

On Mon, 07 Jan 2008 00:42:01 +0000, Grant Edwards wrote:

>> If you want to delete them, you still have to do the rest of the job
>> yourself.
>
> Nonsense.
>
> How is this not doing what the OP asks?
>
> grep -v pattern infile >outfile; mv outfile infile

It isn't deleting lines. As abstractions go, it comes pretty close, but
just try it on a disk with insufficient free space for the temporary
outfile and watch it break.


--
Steven