Python Command-Line Arguments Tutorial – sys.argv and argparse Explained

Python Command-Line Arguments

When running Python programs, we usually execute them like this:

python script.py

But sometimes we need to pass extra information while running the script.

This is done using Command-Line Arguments.

They allow users to pass input values directly when executing a program.

---

What are Command-Line Arguments?

Command-line arguments are values passed to a Python script from the terminal.

Example:

python app.py John 25

Here:

• John → first argument
• 25 → second argument

---

Why Use Command-Line Arguments?

They help to:

• Pass input without modifying code
• Build CLI tools
• Automate tasks
• Create flexible programs
• Improve user interaction

---

1. Using sys.argv

Python provides the sys module to handle command-line arguments.

---

Example

import sys

print("Script Name:", sys.argv[0])
print("Arguments:", sys.argv[1:])

---

Run Command

python script.py John 25

---

Output

Script Name: script.py
Arguments: ['John', '25']

---

Understanding sys.argv

Index	Meaning
0	Script name
1	First argument
2	Second argument

---

Example: Using Arguments

import sys

name = sys.argv[1]
age = sys.argv[2]

print(f"Name: {name}, Age: {age}")

---

Output

Name: John, Age: 25

---

2. Handling Missing Arguments

import sys

if len(sys.argv) < 3:
    print("Usage: python script.py name age")
else:
    name = sys.argv[1]
    age = sys.argv[2]
    print(name, age)

---

3. Using argparse Module (Recommended)

argparse is a powerful module for building CLI applications.

---

Basic Example

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("name")
parser.add_argument("age")

args = parser.parse_args()

print(args.name)
print(args.age)

---

Run Command

python script.py John 25

---

Output

John
25

---

4. Optional Arguments

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("--name")
parser.add_argument("--age")

args = parser.parse_args()

print(args.name)
print(args.age)

---

Run Command

python script.py --name John --age 25

---

5. Default Values

parser.add_argument("--city", default="Unknown")

---

6. Type Conversion

parser.add_argument("--age", type=int)

Now input is automatically converted to integer.

---

7. Help Message

Run:

python script.py --help

Output:

usage: script.py [-h] [--name NAME] [--age AGE]

---

8. Building a Simple CLI Calculator

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("a", type=int)
parser.add_argument("b", type=int)

args = parser.parse_args()

print("Sum:", args.a + args.b)
print("Difference:", args.a - args.b)

---

Run Command

python calc.py 10 5

---

Output

Sum: 15
Difference: 5

---

9. sys.argv vs argparse

Feature	sys.argv	argparse
Ease of use	Simple	Advanced
Validation	No	Yes
Help messages	No	Yes
Type handling	Manual	Automatic
Recommended	Small scripts	Large CLI apps

---

10. Advanced Features of argparse

Store True/False Flags

parser.add_argument("--debug", action="store_true")

---

Mutually Exclusive Arguments

group = parser.add_mutually_exclusive_group()

group.add_argument("--add", action="store_true")
group.add_argument("--remove", action="store_true")

---

Real-World Applications

Command-line arguments are used in:

• Automation scripts
• DevOps tools
• Data processing scripts
• Machine learning training scripts
• API testing tools
• Backup utilities

---

Example: File Processor

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("filename")

args = parser.parse_args()

with open(args.filename, "r") as file:
    print(file.read())

---

Best Practices

• Use argparse for production scripts
• Validate input values
• Provide helpful descriptions
• Use default values where needed
• Keep CLI simple and user-friendly

---

Common Mistakes

Index Error in sys.argv

print(sys.argv[1])

Fix:

Check length first.

---

Forgetting argparse setup

Always create ArgumentParser instance.

---

Summary

Python command-line arguments allow developers to pass input values when running scripts. The sys.argv module is simple, while argparse provides powerful features for building professional CLI applications.

---

Conclusion

Command-line arguments are essential for building flexible and automated Python tools. By mastering sys.argv and argparse, you can create powerful command-line applications for real-world use cases.