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Python Command Line Arguments | FACE Prep

Published on 09 Mar 2020

Similar to the built-in functions raw_input() and input(), Python command-line arguments allow us to get input values from the user in the command prompt during the execution time of the program.


What are Command Line Arguments in Python?


Command-line arguments in python are another way to read user inputs. The user inputs are passed as arguments in the command prompt when executing the code. Python provides built-in modules to accept the user inputs that are passed from the command prompt.


Modules to accept Command Line Arguments


There are 2 different types of modules to accept command line arguments in Python. They are,

  • The sys module
  • The getopt module

Let us discuss these modules in detail.


a) The sys module

  • This module provides much simpler ways to read the command line arguments in python
  • The arguments passed using the 'sys' module can be accessed by using the argv command


Save the below-given code in a notepad file with the name cmd.py.

from sys import *
print('The command line arguments are: ')
for i in argv:
  print(i)
print("Type of argv is:",type(argv))

While executing the above code in the command prompt, pass the input values as shown in the below command.

python cmd.py FACEPrep Python

Here, FACEPrep and Python are the user input values. After execution, you will get the output as shown below.

Output:
The command line arguments are:
cmd.py
FACEPrep
Python
Type of argv is: <class 'list'>

We have passed two arguments but the output is obtained with three arguments. The first argument printed in the output is the file name.

When using 'sys' module, all the command line arguments gets stored in the form of a list. By default, the file name gets stored in argv[0], the first user input value in argv[1] and so on. We can also print the number of arguments using the built-in function len() as shown below.

from sys import *
print("The number of arguments: ", len(argv))
print("The argument list: ", str(argv))
Output:
The number of arguments: 3
The argument list: ['cmd.py', 'FACEPrep', 'Python']

Here, len(argv) gives the number of command-line arguments that have been passed from the command prompt along with the file name.


b) The getopt module

This module helps in parsing through the command-line options and arguments. There are 2 functions and an exception under this module.


i) getopt function

This is used to parse the command line options and parameter list.

Syntax
getopt.getopt(args, shortops, longopts=[])

Where,

  • args denotes the argument list to be parsed, ignoring the first argument (filename)
  • shortops denotes the option definition string for single-character options
  • Longopts denotes a sequence of long style option names such as -noarg, --witharg. This is the extended version of shortops and is optional

Below is an example command to pass arguments when we use getopt module.

python cmd.py hello --caps kyle
Output
HELLO, KYLE!

The option -caps convert the argument list into capital letters.


ii) gnu_getopt function

This is similar to the above function, but the GNU style scanning mode is used by default which means that the option and non-option arguments can be intermixed.

Syntax
getopt.gnu_getopt(args, shortopts, longopts=[])

Exception getopt.GetoptError

This exception is raised when an option in the argument list is not recognizable or when an option that requires an argument is left empty.

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