Insert character at a fixed position of lines

[castironpi]

On Jul 26, 10:02 pm, alex23 <wuwe... at gmail.com> wrote:
> Ugh, and in pointing our your inaccurate code I posted my own:
>
> > >>> f = open('dummy.txt','w')
> > >>> f.write(line = 'this doesn't work')
>
> >   File "<stdin>", line 1
> >     f.write(line = 'this doesn't work')
> >                                ^
> > SyntaxError: invalid syntax
>
> That should be:
>
> >>> f.write(line = "this doesn't work")
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: write() takes no keyword arguments
>
> Sorry about that :)

This is close.

import os
size= os.path.getsize( 'test line_insertion.txt' )
f= open( 'test line_insertion.txt', 'r+' )
linelen= len( f.readline( ) )
f.seek( 20, os.SEEK_SET )
while f.tell( )< size:
    f.write( 'y' )
    f.seek( linelen, os.SEEK_CUR )
f.flush( )
f.close( )

It assumes lines are a constant length (big assumption), and skips one
line's length of characters starting from the 20th character.  It
repeats until current file position is past the original length of the
file.