⚙️ spargear

A powerful yet simple Python library for declarative command-line argument parsing, built on top of argparse. spargear enables elegant, type-safe definitions of CLI arguments and subcommands with minimal boilerplate.

Why spargear?

• ✅ Declarative: Define your CLI arguments neatly using Python data classes.
• 🚀 Typed and Safe: Leveraging Python typing and dataclasses to ensure type safety and developer productivity.
• 🔧 Flexible: Supports complex argument parsing scenarios, including subcommands and nested configurations.
• 📦 Minimal Dependencies: Pure Python, built directly upon the reliable argparse module.
• 🎯 Modern API: Beautiful @subcommand or @subcommandclass decorator for intuitive subcommand definition.

Installation

Install with pip:

pip install spargear

Quick Start

Define your arguments:

from typing import Annotated, Optional

from spargear import BaseArguments


class MyArgs(BaseArguments):
    input_file: Annotated[Optional[str], "-i", "--input"] = None
    """Input file path"""
    verbose: Annotated[bool, "-v", "--verbose"] = False
    """Enable verbose output"""


# Parse the command-line arguments
args = MyArgs()

# Access the parsed arguments
input_file: Optional[str] = args.input_file
# input_file: str | None = args.input_file.value
verbose: bool = args.verbose  # store_true `-v` or `--verbose` to True
print(f"Input file: {input_file}")
print(f"Verbose mode: {verbose}")

Run your CLI:

python app.py --input example.txt --verbose

Subcommands with @subcommand or @subcommandclass decorator

The easiest and most intuitive way to define subcommands is using the @subcommand or @subcommandclass decorator:

from spargear import BaseArguments, subcommandclass


class GitCLI(BaseArguments):
    """A Git-like CLI tool."""

    @subcommandclass(help="Initialize a new repository")
    class Init(BaseArguments):
        """Initialize a new Git repository."""

        NAME: str

    @subcommandclass(help="Record changes to the repository")
    class Commit(BaseArguments):
        """Record changes."""

        message: str
        amend: bool = False


# Parse and use
args = GitCLI()

# Access the active subcommand
args.inspect(GitCLI.Init, lambda init_args: print(f"Initializing project: {init_args.NAME}"))
args.inspect(
    GitCLI.Commit,
    lambda commit_args: print(f"Committing: {commit_args.message} (amend: {commit_args.amend}"),
)

Run your CLI:

python app.py init my_project
python app.py commit -m "Initial commit" --amend

@subcommand Features

The @subcommand decorator automatically:

• Extracts the subcommand name from the method name (or use name= parameter to override)
• Uses the method's docstring as help text and description
• Treats the method as a factory that returns the argument class
• No need for @staticmethod - the decorator handles it automatically!

class MyApp(BaseArguments):
    @subcommand()  # Name will be "serve", help from docstring
    def serve():
        """Start the development server.
        
        Launches a local development server on the specified port
        with hot reload capabilities.
        """
        return ServeArguments
    
    @subcommand(name="db-migrate", help="Run database migrations")
    def database_migrate():  # Custom name overrides method name
        return MigrateArguments

Advanced Usage

Manual SubcommandSpec (Alternative approach)

For more control or complex scenarios, you can still use SubcommandSpec directly:

from spargear import BaseArguments, SubcommandSpec, ArgumentSpec


class InitArgs(BaseArguments):
    name: ArgumentSpec[str] = ArgumentSpec(["name"], help="Project name")


class CommitArgs(BaseArguments):
    message: ArgumentSpec[str] = ArgumentSpec(["-m"], required=True, help="Commit message")


class GitCLI(BaseArguments):
    init = SubcommandSpec("init", InitArgs, help="Initialize a new repository")
    commit = SubcommandSpec("commit", CommitArgs, help="Commit changes")

Note: The @subcommand decorator is the recommended approach for most use cases as it provides a cleaner, more maintainable syntax.

Default Factories

Generate dynamic values at parse time:

import uuid
from datetime import datetime
from spargear import BaseArguments, ArgumentSpec


class AppConfig(BaseArguments):
    # Method 1: Direct callable assignment (auto-detected)
    session_id: str = lambda: str(uuid.uuid4())
    
    # Method 2: Explicit ArgumentSpec with default_factory
    log_file: ArgumentSpec[str] = ArgumentSpec(
        ["--log-file"],
        default_factory=lambda: f"log_{datetime.now().strftime('%Y%m%d_%H%M%S')}.txt",
        help="Log file path"
    )
    
    name: str = "myapp"  # Regular default value


config = AppConfig()
print(f"Session ID: {config.session_id}")  # Unique UUID each time
print(f"Log file: {config.log_file.unwrap()}")  # Timestamp-based filename

Configuration Management

Save and load configurations:

from spargear import BaseArguments
from typing import List


class ServerConfig(BaseArguments):
    host: str = "localhost"
    port: int = 8080
    debug: bool = False
    allowed_hosts: List[str] = ["127.0.0.1", "localhost"]


# Create and configure
config = ServerConfig(["--port", "3000", "--debug"])

# Convert to dataclass
config_dc = config.to_dataclass()
print(f"Dataclass: {config_dc}")

# Save to JSON
config.save_config("server.json")

# Load from JSON with command-line overrides
loaded_config = ServerConfig.load_config("server.json", args=["--port", "9000"])
print(f"Port: {loaded_config.port}")  # 9000 (overridden)
print(f"Debug: {loaded_config.debug}")  # True (from file)

# Update from dictionary
config.update_from_dict({"host": "0.0.0.0", "port": 5000})

API Reference

@subcommand Decorator

@subcommand(name=None, help="", description=None, argument_class=None)
def method_name():
    return ArgumentClass

Parameters:

• name (optional): Override the subcommand name (defaults to method name)
• help (optional): Brief help text (defaults to first line of docstring)
• description (optional): Detailed description (defaults to rest of docstring)
• argument_class (optional): Directly specify the argument class instead of using the factory

BaseArguments Methods

Serialization

• to_dict() -> Dict[str, Any] - Convert to dictionary
• to_json() -> str - Serialize to JSON
• to_pickle() -> bytes - Serialize to pickle format
• to_dataclass() - Convert to dataclass instance

Deserialization

• from_dict(data, args=None) - Create from dictionary
• from_json(json_data, args=None) - Create from JSON string/file
• from_pickle(file_path, args=None) - Create from pickle file

Configuration Management

• save_config(file_path, format="json") - Save configuration
• load_config(file_path, format=None, args=None) - Load configuration
• update_from_dict(data) - Update current instance

Subcommand Access

• last_subcommand - Get the active subcommand instance (if any)

ArgumentSpec Features

Default Factories

ArgumentSpec(
    name_or_flags=["--arg"],
    default_factory=lambda: generate_value(),  # Called at parse time
    help="Description"
)

Compatibility

• Python 3.8+

License

MIT