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 filedef 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 moduleimport os# Function to read# last N lines of the filedef 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 filedef 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
