How to read specific lines from a File in Python?

Text files are composed of plain text content. Text files are also known as flat files or plain files. Python provides easy support to read and access the content within the file. Text files are first opened and then the content is accessed from it in the order of lines. By default, the line numbers begin with the 0th index. There are various ways to read specific lines from a text file in python, this article is aimed at discussing them. 

Method 1: fileobject.readlines()

A file object can be created in Python and then readlines() method can be invoked on this object to read lines into a stream. This method is preferred when a single line or a range of lines from a file needs to be accessed simultaneously. It can be easily used to print lines from any random starting index to some ending index. It initially reads the entire content of the file and keep a copy of it in memory. The lines at the specified indices are then accessed. 

Example:

# open the sample file used
file = open('test.txt')

# read the content of the file opened
content = file.readlines()

# read 10th line from the file
print("tenth line")
print(content[9])

# print first 3 lines of file
print("first three lines")
print(content[0:3])

Output 

tenth line
 

This is line 10.

first three lines
 

This is line 1.This is line 2.This is line 3.

Method 2: linecache package 

The linecache package can be imported in Python and then be used to extract and access specific lines in Python. The package can be used to read multiple lines simultaneously. It makes use of cache storage to perform optimization internally. This package opens the file on its own and gets to the particular line. This package has getline() method which is used for the same. 

Syntax: 

getLine(txt-file, line_number)

Example:

# importing required package
import linecache

# extracting the 5th line
particular_line = linecache.getline('test.txt', 4)

# print the particular line
print(particular_line)

Output :

This is line 5.

Method 3: enumerate()

The enumerate() method is used to convert a string or a list object to a sequence of data indexed by numbers. It is then used in the listing of the data in combination with for loop. Lines at particular indexes can be accessed by specifying the index numbers required in an array. 

Example:

# open a file
file = open("test.txt")

# lines to print
specified_lines = [0, 7, 11]

# loop over lines in a file
for pos, l_num in enumerate(file):
    # check if the line number is specified in the lines to read array
    if pos in specified_lines:
        # print the required line number
        print(l_num)

Output

This is line 1.
This is line 8.
This is line 12.