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@pelson pelson/index.ipynb
Created Jan 20, 2019

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