[Tutor] Extracting lines in a file

bob gailer bgailer at gmail.com
Tue Apr 6 15:11:46 CEST 2010

On 4/6/2010 2:16 AM, ranjan das wrote:
> Hi,
> I am new to python, and specially to file handling.
> I need to write a program which reads a unique string in a file and 
> corresponding to the unique string, extracts/reads the n-th line (from 
> the line in which the unique string occurs).
> I say 'n-th line' as I seek a generalized way of doing it.
> For instance lets say the unique string is "documentation" (and 
> "documentation" occurs more than once in the file). Now, on each 
> instance that the string "documentation" occurs in the file,  I want 
> to read the 25th line (from the line in which the string 
> "documentation" occurs)

> Is there a goto kind of function in python?

Others have offered linecache and seek.

The simplest generic solution is:

lines_to_be_read = []
for lineno, line in enumerate(open(filename)):
   if "documentation" in line:
     lines_to_be_read.append(lineno + 25)
   if lineno in lines_to_be_read:
     # process this line

Bob Gailer
Chapel Hill NC

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