Flags & Options: -help Me Help You

Welcome to Part 2 of the argparse mastery series — written from real CLI-building experience.

Let’s start with the essential tools: boolean flags, default values, type safety, and clear help messages.

🛠️ What You’ll Learn

• Boolean flags like --debug, --verbose, etc.
• Setting default values
• Type conversion (int, float, str)
• Writing helpful --help messages
• Making your CLI user-friendly & safe

Why This Matters

A command-line tool without options is like a vending machine with one button.

Flags and options turn your CLI into a full menu — letting users control behavior without touching the code.

• Want more output? Use --verbose
• Need troubleshooting info? Use --debug
• Tweak behavior? Use --rate or --level

Welcome to the pro zone.

What Is a Flag?

A flag is an optional switch that toggles something on or off. It doesn't require a value.

Examples:

• --verbose: show more output
• --debug: print internal details for developers
• --dry-run: simulate action without doing it

They’re usually False by default. If the user includes the flag, it becomes True.

What Is an Option with a Value?

These require extra data, such as:

• --level 3 → sets difficulty
• --rate 0.5 → adjusts speed

You can define:

• The expected data type (type=int, type=float, etc.)
• A fallback value with default=

Sample Code

import argparse

parser = argparse.ArgumentParser(description="Demo: Flags and Options in argparse")

# Boolean flags
parser.add_argument("--debug", action="store_true", help="Enable debug mode")
parser.add_argument("--verbose", action="store_true", help="Enable verbose output")

# Options with values
parser.add_argument("--level", type=int, default=1, help="Set difficulty level (default: 1)")
parser.add_argument("--rate", type=float, default=0.5, help="Set processing rate (default: 0.5)")

args = parser.parse_args()

# Logic based on flags
if args.debug:
    print("[DEBUG] Debug mode is ON")

if args.verbose:
    print("[VERBOSE] Verbose output is ON")

# Always show these
print("Level:", args.level)
print("Rate:", args.rate)

🧪 Try It Out

$ python3 tool.py --verbose --level 3 --rate 1.2
[VERBOSE] Verbose output is ON
Level: 3
Rate: 1.2

$ python3 tool.py
Level: 1
Rate: 0.5

$ python3 tool.py --debug
[DEBUG] Debug mode is ON
Level: 1
Rate: 0.5

📌 Quick Summary of Each Option

• --debug: Boolean, prints debug details if enabled
• --verbose: Boolean, prints extra output
• --level: Integer, sets difficulty (default = 1)
• --rate: Float, controls speed (default = 0.5)

💬 Pro Tips

• ✅ Use action="store_true" for flags you want OFF by default
• ✅ Use type= to force input to match the expected type
• ✅ Use default= to keep your app safe when users forget values
• ✅ Always write help= text — that’s your CLI’s first impression

💻 CLI Help Output

$ python3 tool.py --help
usage: tool.py [options]

Demo: Flags and Options in argparse

optional arguments:
  --debug         Enable debug mode
  --verbose       Enable verbose output
  --level LEVEL   Set difficulty level (default: 1)
  --rate RATE     Set processing rate (default: 0.5)

💻 Real-Life Use Case

This kind of base setup is great for:

• Batch processing files
• Transforming log entries
• Controlling automation scripts

In short: any script can become a CLI tool with just these building blocks.

🏁 Coming Up Next: Part 3 — Validation & Errors

We’ll cover:

• Type enforcement
• Input validation
• Safe defaults
• Defensive CLI design

Because broken input shouldn’t break your script.