Slowness in compile()/eval()

[Alex Martelli]

"Pedro Rodriguez" <pedro_rodriguez at club-internet.fr> wrote in message
news:pan.2002.01.17.14.19.06.888663.2742 at club-internet.fr...
> Hello,
>
> while investigating on lazy evaluations, I found these strange
> results while doing some timings. I expected that using :
>     d = eval(code)
> where
>     code = compile("i+i", '<string>', 'eval')

This uses a GLOBAL variable named 'i'...

> should give an execution time of the same magnitude as :
>     d = f(i)
> where
>     def f(i): return i+i

...while this uses a LOCAL one.


> it turns out to be almost 4x slower.
>
> Investigating with the dis module, I noticed that the result of
> compile uses LOAD_NAME where a function uses LOAD_FAST. Is this
> due to local namespace lookup ?

Right.  Local is faster than global, exactly because it can
be loaded without a lookup (LOAD_NAME).

Change your function to:
    def f(): return i+i
and the time difference should just about go away.


> It turns out that using something like (with some tweaks) :
>     code = compile("def f(i):\n return i*i\n", '<string>', 'exec')
>     eval(code)
> gives the expected computation time.
>
> I think I must be missing something on evaluation, but what ?

I don't think you're necessarily missing anything.


Alex