Python – Reading last N lines of a file

Prerequisite: Read a file line-by-line in Python
Given a text file fname, a number N, the task is to read the last N lines of the file.
As we know, Python provides multiple in-built features and modules for handling files. Let’s discuss different ways to read last N lines of a file using Python. 
File:

Method 1: Naive approach
In this approach, the idea is to use a negative iterator with the readlines() function to read all the lines requested by the user from the end of file.

Python3

# Python implementation to
# read last N lines of a file

# Function to read
# last N lines of the file
def LastNlines(fname, N):
    # opening file using with() method
    # so that file get closed
    # after completing work
    with open(fname) as file:

        # loop to read iterate
        # last n lines and print it
        for line in (file.readlines() [-N:]):
            print(line, end ='')


# Driver Code:
if __name__ == '__main__':
    fname = 'File1.txt'
    N = 3
    try:
        LastNlines(fname, N)
    except:
        print('File not found'

Output:

Eighth line
Ninth line
Tenth line

Method 2: Using OS module and buffering policy
In this approach, the idea is to work on the buffering policy in the python. A buffer stores a part of data received from a file stream of the operating system for a time period it is used and then more data comes in. 
The buffer size determines the size of the data that can be stored at a time until it is used. We have the option to pass an integer to buffering in order to set buffering policy and if we do not specify any policy then the size of the buffer depends upon the device’s block size. Usually, the buffer is 4096 or 8192 bytes long. In this approach size of the buffer is 8192 bytes.
Moreover, the st_size attribute of os.stat() method in the OS module is used to represent the size of the file in bytes.
Below is the implementation of the above approach.

Python3

# Python implementation to
# read last N lines of a file
# Using OS module and buffering policy

# importing os module
import os

# Function to read
# last N lines of the file
def LastNlines(fname, N):
    # taking buffer size of 8192 bytes
    bufsize = 8192

    # calculating size of
    # file in bytes
    fsize = os.stat(fname).st_size

    iter = 0

    # opening file using with() method
    # so that file get closed
    # after completing work
    with open(fname) as f:
        if bufsize > fsize:

            # adjusting buffer size
            # according to size
            # of file
            bufsize = fsize-1

            # list to store
            # last N lines
            fetched_lines = []

            # while loop to
            # fetch last N lines
            while True:
                iter += 1

                # moving cursor to
                # the last Nth line
                # of file
                f.seek(fsize-bufsize * iter)

                # storing each line
                # in list upto
                # end of file
                fetched_lines.extend(f.readlines())

                # halting the program
                # when size of list
                # is equal or greater to
                # the number of lines requested or
                # when we reach end of file
                if len(fetched_lines) >= N or f.tell() == 0:
                        print(''.join(fetched_lines[-N:]))
                        break

# Driver Code:
if __name__ == '__main__':

    fname = 'File1.txt'
    N = 3

    try:
        LastNlines(fname, N)
    except:
        print('File not found')

Output:

Eighth line
Ninth line
Tenth line

Method 3: Through Exponential search
In this method, the idea is to use Exponential Search algorithm which is generally used for searching sorted, unbounded or infinite lists. To get information about exponential search click here.
This approach uses assert statement which acts as a debugging tool to checks a condition. The program will continue to execute if the given statement is true otherwise, it generates an AssertionError exception. To get more details of assert statements click here.
Click here to get familiar with different kinds of use of seek() method.
Below is the implementation of the above approach.

Python3

# Python implementation to
# read last N lines of a file
# through Exponential search

# Function to read
# last N lines of the file
def LastNlines(fname, N):

    # assert statement check
    # a condition
    assert N >= 0

    # declaring variable
    # to implement
    # exponential search
    pos = N + 1

    # list to store
    # last N lines
    lines = []

    # opening file using with() method
    # so that file get closed
    # after completing work
    with open(fname) as f:

        # loop which runs
        # until size of list
        # becomes equal to N
        while len(lines) <= N:

            # try block
            try:
                # moving cursor from
                # left side to
                # pos line from end
                f.seek(-pos, 2)

            # exception block
            # to handle any run
            # time error
            except IOError:
                f.seek(0)
                break

            # finally block
            # to add lines
            # to list after
            # each iteration
            finally:
                lines = list(f)

            # increasing value
            # of variable
            # exponentially
            pos *= 2

    # returning the
    # whole list
    # which stores last
    # N lines
    return lines[-N:]

# Driver Code:
if __name__ == '__main__':
    fname = 'File1.txt'
    N = 3
    try:
        lines = LastNlines(fname, N)
        for line in lines:
            print (line, end ='')
    except:
        print('File not found')

Output:

Eighth line
Ninth line
Tenth line