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 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This notebook demonstrates an example of a model, where the `evaluate` method dispatches to something not unit aware (like C code), where the input and output needs to be in fixed units."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import numpy as np\n",
    "\n",
    "import astropy.units as u\n",
    "from astropy.modeling import models as m\n",
    "from astropy.modeling import Model\n",
    "from astropy.utils.decorators import wraps"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Define a decorator to create a quantity object from the return value of a function."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 86,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class CreateReturnQuantity(object):\n",
    "    \"\"\"\n",
    "    Create a quantity of the return value and the return annotation as a unit.\n",
    "    \"\"\"\n",
    "    @classmethod\n",
    "    def as_decorator(cls, func=None, **kwargs):\n",
    "        self = cls(**kwargs)\n",
    "        if func is not None and not kwargs:\n",
    "            return self(func)\n",
    "        else:\n",
    "            return self\n",
    "    \n",
    "    def __init__(self, return_unit=None):\n",
    "        self.return_unit = return_unit\n",
    "    \n",
    "    def __call__(self, func):\n",
    "        if hasattr(func, '__annotations__'):\n",
    "            return_unit = func.__annotations__['return']\n",
    "        elif self.return_unit:\n",
    "            return_unit = self.return_unit\n",
    "        else:\n",
    "            raise ValueError(\"The return unit must be specified either as a\"\n",
    "                             \" function annotation or as an argument to the decorator.\")\n",
    "            \n",
    "        @wraps(func)\n",
    "        def wrapper(*args, **kwargs):\n",
    "            return u.Quantity(func(*args, **kwargs), unit=return_unit)\n",
    "        \n",
    "        return wrapper\n",
    "    \n",
    "create_return_quantity = CreateReturnQuantity.as_decorator"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Our dummy function that `evaluate` will use."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 77,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def some_unitless_function(ham, eggs):\n",
    "    \"\"\"\n",
    "    This function does something in code that has no idea what a unit is.\n",
    "    \n",
    "    Parameters\n",
    "    ----------\n",
    "    ham : `float`\n",
    "        ham in units of kg\n",
    "        \n",
    "    eggs : `float`\n",
    "        eggs in units of m\n",
    "    \n",
    "    Returns\n",
    "    -------\n",
    "    Ni : `float`\n",
    "       The result of ham combined with eggs. In units of J.\n",
    "    \"\"\"\n",
    "    return np.array(ham) + np.array(eggs)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A model:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 91,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "class Spam(Model):\n",
    "    inputs = ('ham', 'eggs')\n",
    "    outputs = ('Ni',)\n",
    "    \n",
    "    @property\n",
    "    def input_units(self):\n",
    "        return u.kg, u.m\n",
    "    \n",
    "    @classmethod\n",
    "    @create_return_quantity\n",
    "    def evaluate(cls, x, y) -> u.J:\n",
    "        # Compatibility with these units is ensured by `input_units`\n",
    "        # however, this no longer supports non-quantity input.\n",
    "        x = x.to(u.kg)\n",
    "        y = y.to(u.m)\n",
    "        # Call the external function\n",
    "        return some_unitless_function(x, y)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 88,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "s = Spam()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 89,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/latex": [
       "$0.11 \\; \\mathrm{J}$"
      ],
      "text/plain": [
       "<Quantity 0.11 J>"
      ]
     },
     "execution_count": 89,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s(10*u.g, 10*u.cm)"
   ]
  }
 ],
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