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Python - Command Line Arguments

Introduction

Command Line Arguments are the values passed to a Python program at the time of execution.

These values can be accessed inside the program using the argv list available in the sys module.

What is argv?

argv is a list that stores all command line arguments.

  • argv is a list, not an array.
  • It is available in the sys module.

Example - Command Line Arguments

D:\Python_classes> py test.py 10 20 30

Here:

  • 10
  • 20
  • 30

are the command line arguments.

Accessing Command Line Arguments

Command line arguments are available through argv in the sys module.

Import argv

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main.py
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Important Note

  • argv[0] represents the program name.
  • argv[1] represents the first command line argument.
  • argv[2] represents the second command line argument.

Program to Check the Type of argv

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main.py
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Program to Display Command Line Arguments

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main.py
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Execution

D:\Python_classes> py test.py 10 20 30

Output

The Number of Command Line Arguments: 4
The List of Command Line Arguments:
['test.py', '10', '20', '30']

Command Line Arguments one by one:
test.py
10
20
30

Program to Find the Sum of Command Line Arguments

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main.py
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Execution

D:\Python_classes> py test.py 10 20 30 40

Output

The Sum: 100

Important Note - Space Separator

By default, command line arguments are separated by spaces.

If an argument contains spaces, it must be enclosed in double quotes.

Example 1

D:\Python_classes> py test.py Sunny Leone

Output

Sunny

Example 2

D:\Python_classes> py test.py 'Sunny Leone'

Output

'Sunny

Example 3

D:\Python_classes> py test.py "Sunny Leone"

Output

Sunny Leone

Important Note - Type Conversion

All command line arguments are stored as strings.

Use type casting to convert them into the required data type.

Example - String vs Integer

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main.py
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Execution

D:\Python_classes> py test.py 10 20

Output

1020
30

Important Note - Invalid Index

If you try to access an index that does not exist, Python raises an IndexError.

Example - Invalid Index

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main.py
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Execution

D:\Python_classes> py test.py 10 20

Output

IndexError: list index out of range

argparse Module

Python provides the argparse module for advanced command line argument handling.

It is used to:

  • Parse command line arguments.
  • Display help messages when the user enters incorrect input.

Real World Usage

Command line arguments are commonly used in:

  • Automation scripts
  • System administration
  • Data processing programs
  • Backup utilities
  • Python tools and utilities

Important Notes

  • argv is a list, not an array.
  • Command line arguments are available through the sys module.
  • argv[0] contains the program name.
  • argv[1:] contains all command line arguments except the program name.
  • All command line arguments are stored as strings.
  • Use type casting such as int() and float() when required.
  • If an argument contains spaces, enclose it in double quotes.
  • Accessing an invalid index raises IndexError.
  • The argparse module provides advanced command line argument parsing.

Quick Summary

Topic Description
argv List containing command line arguments.
Module sys
argv[0] Program name.
argv[1] First command line argument.
argv[1:] All command line arguments except the program name.
Data Type String
Type Conversion Use int(), float(), etc.
Invalid Index Raises IndexError.
Advanced Parsing argparse module.

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