Question

Python

Chet is trying to create a tail program for his windows machine so he can see the last N lines of a file or list of files. He wants the program to be usable by his friends and family so when an argument does not get passed to the program it should display a usage message. This is shown by the parser object by default but it's not doing it for him. The code works just not this one aspect of it. How can he fix it so that the parser object does it on it's own?

Only one thing needs to be changed in the code. No need to add any additional print statements or conditions. The output should look like

usage: tail [-h] [-n LINES] files [files ...]

tail: error: too few arguments

Code:

import argparse as arg
import linecache as cache
import os.path

parser = arg.ArgumentParser(prog="tail", description="""Print the last lines of 
a file to standard output.  With more than one file, 
precede each with a header giving the file name.""")

parser.add_argument('files', nargs='*', help='A list of files to be tailed.')
parser.add_argument('-n', '--lines', type=int, dest='lines', default=10, 
                    help='The number lines to display from the bottom.')

arguments = parser.parse_args()
lines_to_print = arguments.lines
files_to_open = arguments.files

for filepath in files_to_open:
    print("FILE: {}".format(filepath))
    if os.path.exists(filepath) and os.path.isfile(filepath):
        with open(filepath, 'r') as tailed_file:
            filesize = len(tailed_file.readlines())
        for i in range(lines_to_print):
            print(cache.getline(filepath, i).rstrip('\n'))

Answer 1

The solution to the above question is given below with screenshot of output.

=====================================================

I kept the logic simple and i have tested all outputs.

I Have added comments in program so that it can be easily

understood.

If there is anything else do let me know in comments.

=====================================================

============ CODE TO COPY ===============================

tail.py

import argparse as arg
import linecache as cache
import os.path
import sys

if len( sys.argv ) == 1 :
                print("usage: tail [-h] [-n LINES] files [files ...]")
                print("tail: error: too few arguments")
                sys.exit(1)

parser = arg.ArgumentParser(prog="tail", description="""Print the last lines of 
a file to standard output.  With more than one file, 
precede each with a header giving the file name.""")

parser.add_argument('files', nargs='*', help='A list of files to be tailed.')
parser.add_argument('-n', '--lines', type=int, dest='lines', default=10, 
                    help='The number lines to display from the bottom.')

arguments = parser.parse_args()
lines_to_print = arguments.lines
files_to_open = arguments.files

for filepath in files_to_open:
    print("FILE: {}".format(filepath))
    if os.path.exists(filepath) and os.path.isfile(filepath):
        with open(filepath, 'r') as tailed_file:
            filesize = len(tailed_file.readlines())
        for i in range(lines_to_print):
            print(cache.getline(filepath, i).rstrip('\n'))

saved main.py import argparse as arg 1 import linecache as cache 2 import os.path import sys 4 5 if len( sys.argv) print("usage: tail [-h] [-n LINES] files [files ...]") print("tail: error: too few arguments") sys.exit(1) = 1 : 6 7 10 arg.ArgumentParser(prog= "tail", description-""" Print the last lines of With more than one file, 11 parser a file to standard output. 12 precede each with a header giving the file name.") 13 14 ument('files', nargs-* elp-'A list of files to be tailed. ) parser.add argument(-n', ' --lines', type-int, dest-'lines', default-10, help-'The number lines to display from the bottom. ) parser.add 15 16 17 18 arguments parser.parse args) lines to print arguments.lines files to open arguments.files 19 28 21 22 for filepath in files to_open: 23 print("FILE: ".format (filepath)) if os.path.exists (filepath) and os.path.isfile (filepath): with open(filepath, 'r') filesize for i in range(lines to_print): print(cache.getline (filepath, i).rstrip'\n')) 24 25 as tailed file: 26 len(tailed_file.readlines ()) 27 28 29

=====================================================

Output :

c:NUsersDe-1\Desktop>python tail.py usage tail [-h] [-n LINES) files [files ...] tail: error: too few argunents C:UserDe-1\Desktop> CNUsersDeu-1\Desktop> CENUsersDeu-1\Desktop> CENUsersDeu-1\Desktop> CENUsers Dev-1\Desktop>pyt hon tail.py usage tail [-h] [-n LINES) files [files ...] tail: error: too few argunents CENUsersDeu-1\Desktop> c:NIlserg\Deu-1 Deskton>