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

Guido van Rossum guido@python.org
Thu, 27 Mar 2003 22:23:22 -0500

Hi Raymond. Too bad you couldn't make it to the conference! We're all having a great time on and off the GWU premises. I used your "more zen" on a slide in my keynote.

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. E.g. suppose module A contains:

def f(a): return len(a)

and module B contains:

import A A.len = lambda a: len(a) or 1 # evil len()

The assignment to A.len would be forbidden.

OTOH this:

import random if random.random() >= 0.5: len = 42 def f(): return len

will always be allowed and mean what it currently means.

The difference is that in the first module, analysis of module A does not reveal that len is shadowed; OTOH in the second example, analyzing just the module's code shows that len may be a global built-in. This is important because a programmer shouldn't have to know the names of built-in objects she doesn't use (also important because in a future version of the language, a name you've picked for a global may become a builtin).

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 PyOject_Size() 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. But it cannot be required to look at all other modules (since those other modules may not even have been written!).

Hope this helps.

BTW this idea is quite old; I've described it a few years ago under a subject something like "low-hanging fruit".

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