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Command Line Interface Programming in Python

Last Updated : 07 Mar, 2022
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This article discusses how you can create a CLI for your python programs using an example in which we make a basic "text file manager". 
Let us discuss some basics first. 
 

What is a Command Line Interface(CLI)?


A command-line interface or command language interpreter (CLI), also known as command-line user interface, console user interface, and character user interface (CUI), is a means of interacting with a computer program where the user (or client) issues commands to the program in the form of successive lines of text (command lines).(Wiki)
Advantages of CLI: 
 

  • Requires fewer resources
  • Concise and powerful
  • Expert-friendly
  • Easier to automate via scripting


 

Why to use CLI in your python program?


 

  • Having even just a very basic command-line interface (CLI) for your program can make everyone's life easier for modifying parameters, including programmers, but also non-programmers.
  • A CLI for your program can also make it easier to automate running and modifying variables within your program, for when you want to run your program with a cronjob or maybe an os.system call.


Now, let us start making our "Text file manager". Here, we will be using a built-in python library called Argparse.
About Argparse: 
 

  • It makes it easy to write user-friendly command-line interfaces.
  • The program defines what arguments it requires, and argparse will figure out how to parse those out of sys.argv.
  • The argparse module also automatically generates help and usage messages and issues errors when users give the program invalid arguments.


Ok let's start with a really basic program to get a feel of what argparse does.
 

Python 
# importing required modules
import argparse

# create a parser object
parser = argparse.ArgumentParser(description = "An addition program")

# add argument
parser.add_argument("add", nargs = '*', metavar = "num", type = int, 
                     help = "All the numbers separated by spaces will be added.")

# parse the arguments from standard input
args = parser.parse_args()

# check if add argument has any input data.
# If it has, then print sum of the given numbers
if len(args.add) != 0:
    print(sum(args.add))

Let us go through some important points related to above program: 
 

  • First of all, we imported the argparse module.
  • Then, created a ArgumentParser object and also provided a description of our program.
  • Now, we can fill up our parser object with information by adding arguments. In this example, we created an argument add. A lot of arguments can be passed to the add_argument function. Here I explain the ones I have used in above example: 
    argument 1: ("add") It is nothing but the name of the argument. We will use this name to access the add arguments by typing args.add. 
    argument 2: (nargs = '*') The number of command-line arguments that should be consumed. Specifying it to '*' means it can be any no. of arguments i.e, from 0 to anything. 
    argument 3: (metavar = 'num') A name for the argument in usage messages. 
    argument 4: (type = int) The type to which the command-line argument should be converted. It is str by default. 
    argument 5: (help) A brief description of what the argument does.
  • Once we have specified all the arguments, it is the time to parse the arguments from the standard command line input stream. For this, we use parse_args() function.
  • Now, one can simply check if the input has invoked a specific argument. Here, we check the length of args.add to check if there is any data received from input. Note that values of an argument are obtained as a list.
  • There are two types of arguments: Positional and Optional. 
    Positional ones are those which do not need any specification to be invoked. Whereas, optional arguments need to be specified by their name first (which starts with '--' sign, '-' is also a shorthand.)
  • One can always use --help or -h optional argument to see the help message. 
    Here is an example (The python script has been saved as add.py): 
     
2
  • Now, let us have a look at another example where our positional argument add is invoked. 
     
  • One more special feature worth mentioning is how argparse issues errors when users give the program invalid arguments. 
     


So, this was a basic example so that you can get comfortable with argparse and CLI concept. Now, let us move on to our "Text file manager" program.
 

Python 
# importing the required modules
import os
import argparse

# error messages
INVALID_FILETYPE_MSG = "Error: Invalid file format. %s must be a .txt file."
INVALID_PATH_MSG = "Error: Invalid file path/name. Path %s does not exist."


def validate_file(file_name):
    '''
    validate file name and path.
    '''
    if not valid_path(file_name):
        print(INVALID_PATH_MSG%(file_name))
        quit()
    else if not valid_filetype(file_name):
        print(INVALID_FILETYPE_MSG%(file_name))
        quit()
    return
    
def valid_filetype(file_name):
    # validate file type
    return file_name.endswith('.txt')

def valid_path(path):
    # validate file path
    return os.path.exists(path)
        
    

def read(args):
    # get the file name/path
    file_name = args.read[0]

    # validate the file name/path
    validate_file(file_name)

    # read and print the file content
    with open(file_name, 'r') as f:
        print(f.read())


def show(args):
    # get path to directory
    dir_path = args.show[0]
    
    # validate path
    if not valid_path(dir_path):
        print("Error: No such directory found.")
        exit()

    # get text files in directory
    files = [f for f in os.listdir(dir_path) if valid_filetype(f)]
    print("{} text files found.".format(len(files)))
    print('\n'.join(f for f in files))
    

def delete(args):
    # get the file name/path
    file_name = args.delete[0]

    # validate the file name/path
    validate_file(file_name)
    
    # delete the file
    os.remove(file_name)
    print("Successfully deleted {}.".format(file_name))
    

def copy(args):
    # file to be copied
    file1 = args.copy[0]
    # file to copy upon
    file2 = args.copy[1]

    # validate the file to be copied
    validate_file(file1)

    # validate the type of file 2
    if not valid_filetype(file2):
        print(INVALID_FILETYPE_MSG%(file2))
        exit()

    # copy file1 to file2
    with open(file1, 'r') as f1:
        with open(file2, 'w') as f2:
            f2.write(f1.read())
    print("Successfully copied {} to {}.".format(file1, file2))


def rename(args):
    # old file name
    old_filename = args.rename[0]
    # new file name
    new_filename = args.rename[1]

    # validate the file to be renamed
    validate_file(old_filename)

    # validate the type of new file name
    if not valid_filetype(new_filename):
        print(INVALID_FILETYPE_MSG%(new_filename))
        exit()

    # renaming
    os.rename(old_filename, new_filename)
    print("Successfully renamed {} to {}.".format(old_filename, new_filename))
def main():
    # create parser object
    parser = argparse.ArgumentParser(description = "A text file manager!")

    # defining arguments for parser object
    parser.add_argument("-r", "--read", type = str, nargs = 1,
                        metavar = "file_name", default = None,
                        help = "Opens and reads the specified text file.")
    
    parser.add_argument("-s", "--show", type = str, nargs = 1,
                        metavar = "path", default = None,
                        help = "Shows all the text files on specified directory path.\
                        Type '.' for current directory.")
    
    parser.add_argument("-d", "--delete", type = str, nargs = 1,
                        metavar = "file_name", default = None,
                        help = "Deletes the specified text file.")
    
    parser.add_argument("-c", "--copy", type = str, nargs = 2,
                        metavar = ('file1','file2'), help = "Copy file1 contents to \
                        file2 Warning: file2 will get overwritten.")
    
    parser.add_argument("--rename", type = str, nargs = 2,
                        metavar = ('old_name','new_name'),
                        help = "Renames the specified file to a new name.")

    # parse the arguments from standard input
    args = parser.parse_args()
    
    # calling functions depending on type of argument
    if args.read != None:
        read(args)
    elif args.show != None:
        show(args)
    elif args.delete !=None:
        delete(args)
    elif args.copy != None:
        copy(args)
    elif args.rename != None:
        rename(args)


if __name__ == "__main__":
    # calling the main function
    main()

After the previous example, the above code seems self explanatory. 
All we did was to add a set of arguments for our file manager program. Note that all these arguments are optional arguments. So, we use some if-elif statements to match the command line input with correct argument type function so that query could be processed. 
Here are a few screenshots which describe the usage of above program: 
 

  • Help message (The python script has been saved as tfmanager.py): 
     
  • Here are examples of operations using the text file manager: 
     


So, this was an example of a simple CLI python program which we made. Many complex CLIs could be easily created by the Argparse module. I hope that these examples will give you a head start in this area.
 


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