Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources] نسخه متنی

Recipe 2.4. Reading a Specific Line from a File

Credit: Luther Blissett

Problem

You want to read from a text file a
single line, given the line number.

Solution

The standard Python library linecache module makes
this task a snap:

import linecache
theline = linecache.getline(thefilepath, desired_line_number)

Discussion

The standard linecache module is usually the
optimal Python solution for this task. linecache
is particularly useful when you have to perform this task repeatedly
for several lines in a file, since linecache
caches information to avoid uselessly repeating work. When you know
that you won't be needing any more lines from the
cache for a while, call the module's
clearcache function to free the memory used for
the cache. You can also use checkcache if the file
may have changed on disk and you must make sure you are getting the
updated version.

linecache reads and caches all of the text file
whose name you pass to it, so, if it's a very large
file and you need only one of its lines, linecache
may be doing more work than is strictly necessary. Should this happen
to be a bottleneck for your program, you may get an increase in speed
by coding an explicit loop, encapsulated within a function, such as:

def getline(thefilepath, desired_line_number):
if desired_line_number < 1: return ''
for current_line_number, line in enumerate(open(thefilepath, 'rU')):
if current_line_number == desired_line_number-1: return line
return ''

The only detail requiring attention is that
enumerate counts from 0, so, since we assume the
desired_line_number argument counts from 1, we need
the -1 in the == comparison.

See Also

Documentation for the linecache module in the
Library Reference and Python in a
Nutshell; Perl Cookbook recipe
8.8.