23 July 2012

Python 3 wonders - Barry BDFL

While writing Nuitka I get to see an absurd amount of CPython code. For a while now, it’s also CPython3.2 that I look at. Checking out __future__ handling, I was surprised the other day though, this really works:

# Python 3.2.3 (default, Jun 25 2012, 23:10:56)
# [GCC 4.7.1] on linux2
>>> from __future__ import barry_as_FLUFL
>>> 1 <> 2
>>> 1 != 2
  File "<stdin>", line 1
    1 != 2
SyntaxError: invalid syntax

It’s new in CPython3, and this the code that makes it possible, from the Python parser:

if (type == NOTEQUAL) {
    if (!(ps->p_flags & CO_FUTURE_BARRY_AS_BDFL) &&
                    strcmp(str, "!=")) {
        err_ret->error = E_SYNTAX;
    else if ((ps->p_flags & CO_FUTURE_BARRY_AS_BDFL) &&
                    strcmp(str, "<>")) {
        err_ret->text = "with Barry as BDFL, use '<>' "
                        "instead of '!='";
        err_ret->error = E_SYNTAX;

Now who would think bad of that, would you? The fun aspect is, that Nuitka will easily supports it. By re-using the Python parser, it works out of the box, I only needed to add the flag value.

For fun, I tried to add a test that confirms - and then notice:

  • It doesn’t really work for CPython3.2 already.

  • The flag is only used for eval and exec and not on the same level, so it’s only inherited.

  • And “2to3” kindly removes that flag silently. It probably should raise an error.

So yeah, oddities in Python3!