[Python-Dev] Fast access to builtins (original) (raw)

Guido van Rossum guido@python.org
Fri, 28 Mar 2003 07:39:04 -0500

Cool. Any chance of getting your keynote slides on the net?

Yes, after the conference.

> > From past rumblings, I gather that Python is moving > > towards preventing builtins from being shadowed. > > You must be misunderstanding. > > The only thing I want to forbid is to stick a name in another > module's globals that would shadow a builtin.

Yes, that is different. Allowing shadows means having to watch out for trees.

Being poetic?

> The idea of forbidding module B in the first example is that the > optimizer is allowed to replace len(a) with a bytecode that calls > PyOjectSize() rather than looking up "len" in globals and builtins. > The optimizer should only be allowed to make this assumption if > careful analysis of an entire module doesn't reveal any possibility > that "len" can be shadowed . . . > BTW this idea is quite old; I've described it a few years ago under a > subject something like "low-hanging fruit".

The fruit is a bit high. Doing a full module analysis means deferring the optimization for a second pass after all the code has already been generated. It's doable, but much harder.

You're stuck in a one-pass compiler mindset. We build a parse tree for the entire module before we start generating bytecode. We already have tools to do namespace analysis for the entire tree (Jeremy added these to implement nested scopes).

def f(x): return len(x) + 10 # knowing whether to optimize this

def g(): global len # when this is allowed len = lambda x: 5 # is a bear The task is much simpler if it can be known in advance that the substitution is allowed (i.e. a module level switch like: fastbuiltins = True).

-1000.

--Guido van Rossum (home page: http://www.python.org/~guido/)