import datetime
from dateutil.relativedelta import relativedelta

def calculate_expiry_date(production_date_str, shelf_life, shelf_unit):
    """Calculates expiry date from production date and shelf life."""
    if not production_date_str or not shelf_life or not shelf_unit:
        return None
    try:
        production_date = datetime.date.fromisoformat(production_date_str)
        life = int(shelf_life)
        if life <= 0: return None # Shelf life must be positive

        if shelf_unit == 'days':
            return production_date + datetime.timedelta(days=life)
        elif shelf_unit == 'months':
            return production_date + relativedelta(months=life)
        else:
            return None # Invalid unit
    except (ValueError, TypeError):
        return None # Invalid date format or shelf life

def convert_volume(amount, from_unit, to_unit):
    """Converts volume between ml and L."""
    if from_unit == to_unit:
        return float(amount)
    try:
        amount = float(amount)
        if from_unit == 'ml' and to_unit == 'L':
            return amount / 1000.0
        if from_unit == 'L' and to_unit == 'ml':
            return amount * 1000.0
        raise ValueError("Invalid unit conversion")
    except (ValueError, TypeError):
         raise ValueError(f"Invalid amount or units for conversion: {amount}, {from_unit}, {to_unit}")