CPython grammar and negative numbers - they only appear after an AST optimisation

index.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "I recently [tweeted](https://twitter.com/pypelson/status/1086869193809633280) about ``int('--1')`` raising an exception, as I was curious whether people would be surprised by this or not.\n",
    "\n",
    "Turns out that people *are* happy with this, but it got me thinking a little bit about the CPython interpreter and its underlying grammar.\n",
    "\n",
    "A quick glance of the AST (Abstract Syntax Tree) produced by the expression ``--1`` shows that Python's grammar does not actually produce negative numbers:  "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Module(Expr(UnaryOp(-, UnaryOp(-, 1))))\n"
     ]
    }
   ],
   "source": [
    "import ast\n",
    "  \n",
    "formatters = {ast.USub: '-', ast.Num: '{n.n}'}\n",
    "\n",
    "def render(node):\n",
    "    chldn = map(render, ast.iter_child_nodes(node))\n",
    "    default_fmt = f'{type(node).__name__}({\", \".join(chldn)})'\n",
    "    fmt = formatters.get(type(node), default_fmt)\n",
    "    return fmt.format(n=node, c=chldn)\n",
    "\n",
    "print(render(ast.parse('--1')))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "I think this is a perfectly reasonable thing to do, else there would need to be some special cases when we have an expression such as ``a-1`` - the tokenizer could easily end up seeing this as ``<VARIABLE><LITERAL>``, and we know that such constructs aren't allowed in the python language."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "For those that are interested, the negative numbers are produced in the AST opmisation step found at https://github.com/python/cpython/blob/v3.7.2/Python/ast_opt.c#L122 (for CPython 3.7.2 anyway)."
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python [default]",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.6.6"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}