Generator vs functools.partial?

[Steven D'Aprano]

On Thu, 21 Jun 2012 21:25:04 +1000, John O'Hagan wrote:

> Sometimes a function gets called repeatedly with the same expensive
> argument:
> 
> def some_func(arg, i):
>     (do_something with arg and i)
> 
> same_old_arg = big_calculation()

Since big_calculation() is only called once, the cost of generating that 
expensive argument is only paid once.

> for i in lots_of_items:
>     some_func(same_old_arg, i)

Passing that same_old_arg to some_func is cheap. Once you've paid the 
cost of producing the value in the first place, there is absolutely no 
difference in cost between passing an "expensive" argument and a "cheap" 
argument.

 
> A simple case like that looks OK, but it can get messy when groups of
> arguments are passed between functions, especially if the arguments are
> used by functions called within the functions they are passed to (by
> other functions!).

I'm not sure what you're trying to say here. Argument passing is cheap. 
Function call overhead is not quite so cheap, but unless you've carefully 
profiled your code and determined that the cost of function overhead is 
significant, you're better off using ignoring it. Likely the actual 
calculation within the function is hundreds or thousands of times more 
expensive than the function call itself.


> Maybe that's a code smell, but it can be cleaned up with:
> 
> import functools
> some_func = functools.partial(some_func, big_calculation()) 
> for i in lots_of_items:
>     some_func(i)

Have you actually measured this to see whether it *actually is* faster, 
or are you just assuming it will be faster?

I expect that at best it will be no faster at all, and quite possibly 
slightly slower. The main reason is that instead of one function call 
(calling some_func directly with two arguments) you now have two (partial 
function called with one argument, which internally calls some_func with 
two). Possibly that internal call is implemented in C and may be a little 
faster than if it were implemented in Python bytecode, but still, two 
calls can't be faster than one.


[...]
> Old news? Thoughts, criticisms, theories?

Premature optimization is the root of all evil.



-- 
Steven