# Python Unit Converter

This utility converts values between different units of measurement (like length and weight) using a dictionary of conversion factors. It demonstrates how to normalize data to a "base unit" to perform conversions between any two supported units.

**Modules Used:**
*   [[programming/python/modules/argparse-module|argparse]]: To handle command-line arguments.

## The Code

Save this as `converter.py`.

```python
import argparse

# Conversion factors relative to a base unit
# Length base: meter (m)
# Weight base: kilogram (kg)
CONVERSIONS = {
    'length': {
        'm': 1.0,
        'km': 1000.0,
        'cm': 0.01,
        'mm': 0.001,
        'mi': 1609.344,
        'yd': 0.9144,
        'ft': 0.3048,
        'in': 0.0254,
    },
    'weight': {
        'kg': 1.0,
        'g': 0.001,
        'mg': 0.000001,
        'lb': 0.45359237,
        'oz': 0.02834952,
    }
}

def convert_units(value, from_unit, to_unit):
    # 1. Identify the category (length, weight, etc.)
    category = None
    for cat, units in CONVERSIONS.items():
        if from_unit in units and to_unit in units:
            category = cat
            break
    
    if category is None:
        print(f"Error: Conversion from '{from_unit}' to '{to_unit}' is not supported.")
        print("Supported units:")
        for cat, units in CONVERSIONS.items():
            print(f"  {cat.capitalize()}: {', '.join(units.keys())}")
        return

    # 2. Perform conversion: Source -> Base -> Target
    factors = CONVERSIONS[category]
    
    # Convert input to base unit (e.g., km -> m)
    base_value = value * factors[from_unit]
    
    # Convert base unit to target unit (e.g., m -> ft)
    final_value = base_value / factors[to_unit]
    
    print(f"{value} {from_unit} = {final_value:.4f} {to_unit}")

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="CLI Unit Converter")
    parser.add_argument("value", type=float, help="Numeric value to convert")
    parser.add_argument("from_unit", help="Source unit (e.g., km, lb)")
    parser.add_argument("to_unit", help="Target unit (e.g., mi, kg)")
    
    args = parser.parse_args()
    
    convert_units(args.value, args.from_unit.lower(), args.to_unit.lower())
```

## Usage

```bash
# Convert 5 Kilometers to Miles
python converter.py 5 km mi

# Convert 150 Pounds to Kilograms
python converter.py 150 lb kg
```

[[programming/python/python]]