Argument Parser in Python

Python argparse - Concise Notes

argparse is a Python library for parsing command-line arguments. It provides a simple interface to define, process, and handle arguments.

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Basic Usage

1. Setup

   import argparse
   parser = argparse.ArgumentParser(description="A simple script")
   

2. Adding Arguments

3. Positional Argument: Required, order matters

   parser.add_argument("filename", help="File to process")
   

4. Optional Argument: Starts with -- or -, order doesn’t matter.

   parser.add_argument("--verbose", action="store_true", help="Enable verbose output")
   parser.add_argument("-n", "--number", type=int, default=1, help="Number of iterations")
   

5. Parsing Arguments

args = parser.parse_args()
print(args.filename)      # Positional argument
print(args.verbose)       # True/False (store_true)
print(args.number)        # Integer value (default: 1)

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Key Argument Settings

1. Positional Arguments:
2. No special prefix.

3. Example: python script.py input.txt

4. Optional Arguments:

5. Use - or --.

6. Example: python script.py --verbose

7. Argument Types:

8. Specify with type (e.g., int, float, str).

9. Example:

   parser.add_argument("--value", type=float, help="A float value")
   

10. Default Values:

11. Use default.

12. Example:

    parser.add_argument("--name", default="User", help="Default name")
    

13. Boolean Flags:

14. Use action="store_true" or action="store_false".

15. Example:

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

---

Example Usage

Script:

import argparse

parser = argparse.ArgumentParser(description="Demo script")
parser.add_argument("filename", help="Input file name")
parser.add_argument("-v", "--verbose", action="store_true", help="Enable verbosity")
parser.add_argument("-n", "--number", type=int, default=10, help="Number of items")
args = parser.parse_args()

print(f"Filename: {args.filename}")
print(f"Verbose: {args.verbose}")
print(f"Number: {args.number}")

Run:

python script.py file.txt --verbose -n 5

Output:

Filename: file.txt
Verbose: True
Number: 5

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Common Methods

1. parser.add_argument(): Define arguments.
2. parser.add_subparsers(): Create subcommands.
3. parser.set_defaults(): Set default functions for arguments.
4. parser.parse_args(): Parse command-line inputs.

---

CLI with subcommands

from argparse import ArgumentDefaultsHelpFormatter, ArgumentParser, RawTextHelpFormatter


class DeepFormatter(ArgumentDefaultsHelpFormatter, RawTextHelpFormatter): ...


def parse_args():
    parser = ArgumentParser(
        description="A simple script to demonstrate argument parsing.",
        formatter_class=DeepFormatter,
    )
    # Top-level subparser
    # dest value is used to check which command was called
    # if `dest=meow`, then `args.meow` will be set to the subcommand called (cache, or optimize)
    subparsers = parser.add_subparsers(dest="command", title="Commands")

    cache_parser = subparsers.add_parser(
        "cache", help="Cache the dataset", formatter_class=DeepFormatter
    )
    cache_parser.add_argument(
        "--cache-dir", type=str, help="Directory to cache the dataset"
    )
    cache_parser.add_argument(
        "--cache-size", type=int, default=100, help="Size of the cache in MB"
    )
    cache_parser.set_defaults(func=handle_cache)
    optimize_parser = subparsers.add_parser(
        "optimize", help="Optimize the dataset", formatter_class=DeepFormatter
    )
    optimize_parser.add_argument(
        "--optimize-level", type=int, default=1, help="Level of optimization (1-3)"
    )
    optimize_parser.set_defaults(func=handle_optimize)
    optimize_parser.add_argument(
        "--optimize-strategy",
        type=str,
        choices=["fast", "balanced", "thorough"],
        default="balanced",
        help="Strategy for optimization",
    )
    return parser.parse_args(), parser

def handle_cache(args):
    print(f"running command: {args.command}")
    print(f"Caching dataset in {args.cache_dir} with size {args.cache_size}MB")

def handle_optimize(args):
    print(f"Optimizing dataset with level {args.optimize_level} and strategy {args.optimize_strategy}")


def main():
    args, parser = parse_args()
    if hasattr(args, "func"):
        args.func(args)
    else:
        parser.print_help()


if __name__ == "__main__":
    main()

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Tips

• Use help for clear CLI documentation.
• Default type is str if not specified.
• Boolean flags (store_true) are ideal for toggles.

For most cases, this basic usage suffices!