[Tutor] Read and write from file

[Kent Johnson]

You can't just overwrite the lines in place because they are different 
lengths. Files aren't that smart - you need to read the file and re-write 
it. You should look at the fileinput module, it makes this kind of 
line-by-line filtering of a file pretty simple. For example, if test.txt 
contains four lines:
1
2
3
4

and I run this program:
import fileinput
for line in fileinput.input('test.txt', inplace=1, backup='.bak'):
     if line == '1\n':
         print 'test'
     elif line == '3\n':
         print '3xx'
     else:
         print line,

then test.txt will contain this:
test
2
3xx
4

and test.bak will have the original contents of test.txt

Kent

At 08:29 PM 8/25/2004 +0200, Øyvind wrote:
>Hello,
>
>    I am trying to read and write from a file, but I cannot get it to do so.
>
>The file has the following lines:
>
>1
>2
>3
>4
>5
>6
>7
>8
>9
>
>I open the file with
>fil = open('d:\\test.txt','rw')
>(I have tried, r, w, w+, r+, a and a+ as well.)
>
>fil.readline()
>gives
>'1\n'
>and so forth.
>
>If I try
>fil.write("test")
>it skips to 4. Why? And why does not the file contain the word test? I
>ahve tried both fil.close and fil.flush without any luck.
>
>What I really want to do is to read the first line, 1, then replace that
>with something else, or remove it. Then read 2, and remove it, then read 3
>and change it and so forth. How can I read, write and modify at the same
>time? So far I have only been able to either read or write, but not both.
>
>Thanks in advance....
>
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