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[2.5.1] Read each line from txt file, replace, and save?

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  [2.5.1] Read each line from txt file, replace, and save? Gilles <nospam@nospam.com> - 2012-09-02 12:19 +0200
    Re: [2.5.1] Read each line from txt file, replace, and save? Gilles <nospam@nospam.com> - 2012-09-02 12:36 +0200
      Re: [2.5.1] Read each line from txt file, replace, and save? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-02 12:02 +0100
      Re: [2.5.1] Read each line from txt file, replace, and save? Terry Reedy <tjreedy@udel.edu> - 2012-09-02 14:04 -0400
        Re: [2.5.1] Read each line from txt file, replace, and save? Gilles <nospam@nospam.com> - 2012-09-05 14:52 +0200

#28259 — [2.5.1] Read each line from txt file, replace, and save?

Hello

This is a newbie question.

I need to read a text file into a variable, loop through each line and
use a regex to substitute some items within the line, and save the
whole variable into a new text file.

This triggers an error when I save the modified variable that contains
all the lines:
==================
import re,sys

f = open("C:\\input.txt", "r")
textlines = f.readlines()
f.close()

for line in textlines:
	#edit each line
	line = "just a test"
	
#rewrite data to new file
log = open('output.sub','w')
#ERROR: argument 1 must be string or read-only character buffer, not
list
log.write(textlines)
log.close()
==================

Should I use another way to read the file, edit each line, and save
the data into a new file?

Thank you.

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#28261

On Sun, 02 Sep 2012 12:19:02 +0200, Gilles <nospam@nospam.com> wrote:
(snip)

Found it:

#rewrite lines to new file
output = open('output.txt','w')

for line in textlines:
	#edit each line
	line = "just a test"
	output.write("%s" % line)

output.close()

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#28262

On 02/09/2012 11:36, Gilles wrote:
> On Sun, 02 Sep 2012 12:19:02 +0200, Gilles <nospam@nospam.com> wrote:
> (snip)
>
> Found it:
>
> #rewrite lines to new file
> output = open('output.txt','w')
>
> for line in textlines:
> 	#edit each line
> 	line = "just a test"
> 	output.write("%s" % line)
>
> output.close()
>

IMHO better practice to use the with statement.  See 
http://docs.python.org/reference/compound_stmts.html#the-with-statement

-- 
Cheers.

Mark Lawrence.

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#28285

On 9/2/2012 6:36 AM, Gilles wrote:
> On Sun, 02 Sep 2012 12:19:02 +0200, Gilles <nospam@nospam.com> wrote:
> (snip)
>
> Found it:
>
> #rewrite lines to new file
> output = open('output.txt','w')
>
> for line in textlines:
> 	#edit each line
> 	line = "just a test"
> 	output.write("%s" % line)
>
> output.close()

If you process each line separately, there is no reason to read them all 
at once. Use the file as an iterator directly. Since line is already a 
string, there is no reason to copy it into a new string. Combining these 
two changes with Mark's suggestion to use with and we have the following 
simple code:

with open('input.txt', 'r') as inp, open('output.txt', 'w') as out:
     for line in inp:
         out.write(process(line))

where for your example, process(line) == 'just a test\n'
(you need explicit line ending for .write())

-- 
Terry Jan Reedy

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#28482

On Sun, 02 Sep 2012 14:04:29 -0400, Terry Reedy <tjreedy@udel.edu>
wrote:
>If you process each line separately, there is no reason to read them all 
>at once. Use the file as an iterator directly. Since line is already a 
>string, there is no reason to copy it into a new string. Combining these 
>two changes with Mark's suggestion to use with and we have the following 
>simple code:

Thanks guys for the suggestion.

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