replacing single line of text

[saad82 at gmail.com]

Simon & Tim, very valuable responses. Thank you much!

Simon Forman wrote:
> saad82 at gmail.com wrote:
> > I want to be able to replace a single line in a large text file
> > (several hundred MB). Using the cookbook's method (below) works but I
> > think the replace fxn chokes on such a large chunk of text. For now, I
> > simply want to replace the 1st line (CSV header) in the file but I'd
> > also like to know a more general solution for any line in the file.
> > There's got a be quick and dirty (and cheap) way to do this... any
> > help?
> >
> > Cookbook's method:
> > output_file.write(input_file.read().replace(stext, rtext))
> >
> > Thanks,
> > Pythonner
>
> The read() method of a file will "read all data until EOF is reached"
> if you don't pass it a size argument.  If your file is "several hundred
> MB" and your RAM is not, you may have some trouble with this.
>
> I don't know how well the replace() method works on tenths-of-a-GB
> strings.
>
> The file object supports line-by-line reads through both the readline()
> and readlines() methods, but be aware that the readlines() method will
> also try to read ALL the data into memory at once unless you pass it a
> size argument.
>
> You can also iterate through the lines in an open file object like so:
>
> for line in input_file:
>     # Do something with the line here.
>
> so, if you know that you want to replace a whole actual line, you could
> do this like so:
>
> for line in input_file:
>     if line == stext:
>         output_file.write(rtext)
>     else:
>         output_file.write(line)
>
>
> Check out the docs on the file object for more info:
> http://docs.python.org/lib/bltin-file-objects.html
> 
> 
> HTH,
> ~Simon