Commandline arguments are arguments provided by the user at runtime and gets executed by the functions or methods in the program. Python provides multiple ways to deal with these types of arguments. The three most common are:
- Using sys.argv
- Using getopt module/li>
- Using argparse module
The Python sys module allows access to command-line arguments with the help of sys module. The python sys module provides functions and variables that can be used to access different parts of the Python Runtime Environment. It allows access to system-specific parameters and functions. To use the sys module we need to import the sys module in our program before running any functions.
There are mainly two functions in the sys module:
sys.argv: gives the list of command-line arguments. For example, sys.argv[0] is the program name.
len(sys.argv): gives the number of command-line arguments.
Example 1 : This program demonstrates the use of sys.argv to fetch the command line arguments and len(sys.argv) gives the total number of command line arguments passed including the name of the python script.
# importing the module import sys # storing the arguments program = sys.argv[0] arg1 = sys.argv[1] arg2 = sys.argv[2] arg3 = sys.argv[3] # displaying the program name print("Program name : " + program) # displaying the arguments print("arg1 : " + arg1) print("arg2 : " + arg2) print("arg3 : " + arg3) print("Number of arguments : ", len(sys.argv)) print(sys.argv) |
Output :
Example 2 : This program demonstrates how command line arguments are passed into a function using sys.argv[index]. The command line arguments at index 1, 2 and 3 are stored into the variables arg1, arg2 and arg3. The variables arg1, arg2 and arg3 are passed to the function defined. However, the command line arguments can be passed directly without storing its value in local variables.
# importing the module import sys # function definition def concat(s1, s2, s3): print(s1 + " " + s2 + " " + s3) # fetching the arguments arg1 = sys.argv[1] arg2 = sys.argv[2] arg3 = sys.argv[3] # calling the function concat(arg1, arg2, arg3) |
Output :
Example 3 : This program demonstrates how command line arguments are passed into a function using sys.argv[index]. The command line arguments at index 1 and 2 are stored into the variables arg1 and arg2. The variables a and b are passed to the function defined. Command line arguments are accepted as strings, hence in order to perform numerical operations it should be converted into the desired numeric type first. For example, in this program the command line argument is converted into integer first and then stored into the variables. However, the command line arguments can be passed directly without storing its value in local variables.
# importing the module import sys # function definition def add(a, b): print("Result", a + b) # fetching the arguments arg1 = int(sys.argv[1]) arg2 = int(sys.argv[2]) # displaying the arguments print(arg1, arg2) # calling the function add(arg1, arg2) |
Output :
Using argparse module
There is a certain problem with sys.argv method as it does not throw any specific error if the argument is not passed or argument of invalid type is passed. The argparse module gracefully handles the absence and presence of parameters. The following examples show the utility of argparse module:
# importing the module import argparse # creating an ArgumentParsert object parser = argparse.ArgumentParser() # fetching the arguments parser.add_argument('number', help = "Enter number to triple it.") args = parser.parse_args() # performing some operation print(args.number * 2) |
Output :
Python by default accepts all command line arguments as string type hence the result is 55 ie. the string gets repeated twice. However, we can specify the data type we are expecting in the program itself so that whenever a command line argument is passed it is automatically converted into expected data type provided it is type compatible.
# importing the module import argparse # creating an ArgumentParsert object parser = argparse.ArgumentParser() # fetching the arguments parser.add_argument('number', help = "Enter number to double.", type = int) args = parser.parse_args() print(args.number * 2) |
Output :
We can look at the errors generated :
This argparse throws specific error unlike the sys module. Any of the modules can be used as per convenience and requirement.
Python provides a getopt module that enables parsing command-line arguments.
Example :
import sys import getopt def full_name(): first_name = None last_name = None argv = sys.argv[1:] try: opts, args = getopt.getopt(argv, "f:l:") except: print("Error") for opt, arg in opts: if opt in ['-f']: first_name = arg elif opt in ['-l']: last_name = arg print( first_name +" " + last_name) full_name() |
Output :
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