[Python-ideas] Changing optimisation level from a script

[Steven D'Aprano]

On Sat, Sep 10, 2016 at 10:04:46AM +1000, Damien George wrote:

> I guess my main question to this list is: if CPython were to add a
> function to change the optimisation level at runtime, what would it
> look like?

I don't think it would look like sys.optimize(flag). At the very least, 
it would have to take *at least* three values:

- no optimizations (the default)
- remove asserts (-O)
- remove asserts and docstrings (-OO)

but even that is very limiting. Why can't I keep asserts, which may be 
useful and important in running code, but remove docstrings, which 
mostly are used interactively? I think the idea of "optimization levels" 
has reached its use-by date and we should start considering separate 
flags for each individual optimization.

In particular, I expect that some time soon somebody will propose at 
least one more optimization:

- remove annotations

and I seem to recall somebody requesting the ability to turn off 
constant folding, although I don't remember why. I can easily imagine 
Python implementations offering flags to control more complex/risky 
optimizations:

- resolve built-in names at compile-time[1]
- inline small functions
- unroll loops

etc. Victor Stinner is (I think) working on bringing some of these to 
CPython, and it seems to me that if they are to be under any 
user-control at all, a simple optimization level counter is not going to 
be sufficient. We're going to need a set of flags.

But it seems to me that the ability to change optimizations at runtime 
would be rather confusing. Shouldn't optimizations, or at least some of 
them, apply on a per-module basis rather than globally? 

I don't even know how to reason about code optimizations if any function 
I run might have changed the optimization level without my knowledge. So 
I think this is a very dubious idea.






[1] Yes, I know that will change the semantics of Python code.


-- 
Steve