#!/usr/bin/env python
# coding: utf-8

# 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.
# 
# Turns out that people *are* happy with this, but it got me thinking a little bit about the CPython interpreter and its underlying grammar.
# 
# 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:  

# In[1]:


import ast
  
formatters = {ast.USub: '-', ast.Num: '{n.n}'}

def render(node):
    chldn = map(render, ast.iter_child_nodes(node))
    default_fmt = f'{type(node).__name__}({", ".join(chldn)})'
    fmt = formatters.get(type(node), default_fmt)
    return fmt.format(n=node, c=chldn)

print(render(ast.parse('--1')))


# 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.

# 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).