How do you handle command-line arguments in Python?

Learn how to process user input from the command line using Python’s argparse module. This is a crucial skill for building flexible and powerful scripts. …

Learn how to process user input from the command line using Python’s argparse module. This is a crucial skill for building flexible and powerful scripts.

Command-line arguments allow users to interact with your Python programs by providing extra information directly when they run the script. Imagine you have a program that converts temperatures. Instead of hardcoding the temperature unit, command-line arguments let the user choose between Celsius or Fahrenheit.

Why are command-line arguments important?

They make your programs:

• Flexible: Users can customize how your program runs without needing to change the code itself.
• Reusable: A single script can handle different tasks depending on the provided arguments.
• Powerful: They enable complex interactions and data processing directly from the command line.

This question is frequently asked in Python interviews because it demonstrates:

1. Understanding of core Python modules: You need to know how argparse (or other argument parsing libraries) work.
2. Ability to write modular code: Handling arguments effectively promotes clean and reusable code structure.

Let’s dive into a practical example using the argparse module:

import argparse

def main():
  # Create an ArgumentParser object
  parser = argparse.ArgumentParser(description='Convert temperatures.') 

  # Add arguments:
  parser.add_argument('temperature', type=float, help='Temperature value to convert')
  parser.add_argument('-u', '--unit', choices=['C', 'F'], default='C', 
                      help='Unit of the input temperature (C or F)')

  # Parse the arguments
  args = parser.parse_args()

  # Perform conversion based on arguments
  if args.unit == 'C':
    converted_temp = (args.temperature * 9/5) + 32
    print(f"{args.temperature}°C is equal to {converted_temp:.2f}°F")
  else: # args.unit == 'F'
    converted_temp = (args.temperature - 32) * 5/9
    print(f"{args.temperature}°F is equal to {converted_temp:.2f}°C")

if __name__ == "__main__":
  main()

Step-by-Step Explanation:

1. Import argparse: This line brings in the necessary tools for handling command-line arguments.

2. Create a parser:

   parser = argparse.ArgumentParser(description='Convert temperatures.') 
   

   This creates an ArgumentParser object, which will help us define and process the arguments. The description string appears when users request help (e.g., by typing python your_script.py -h).

3. Add arguments:

   parser.add_argument('temperature', type=float, help='Temperature value to convert')
   parser.add_argument('-u', '--unit', choices=['C', 'F'], default='C', 
                       help='Unit of the input temperature (C or F)')
   

   • We use add_argument to define each argument:
     • ’temperature’: This is a positional argument, meaning it must be provided directly after the script name. It expects a floating-point number and has a helpful description.
     • ’-u’, ‘–unit’: This is an optional argument. Users can specify it with either -u or --unit.
       • choices=['C', 'F'] restricts the allowed values.
       • default='C' sets Celsius as the default if no unit is provided.

4. Parse arguments:

   args = parser.parse_args()
   

   This line processes the command-line input and stores the parsed arguments in an object called args. You can access each argument’s value using dot notation (e.g., args.temperature, args.unit).

5. Use the arguments: The code then uses the values stored in args to perform the temperature conversion based on the user’s input unit.

Running the Script:

python your_script.py 25 -u C  # Converts 25°C to °F
python your_script.py 77 -u F  # Converts 77°F to °C
python your_script.py 30       # Defaults to converting 30°C to °F

Try modifying this code to handle different types of arguments, add more complex logic, or explore other argument parsing libraries available in Python!