[ python-Feature Requests-1465406 ] Allowing the definition of constants

[SourceForge.net]

Feature Requests item #1465406, was opened at 2006-04-05 18:30
Message generated for change (Comment added) made by rhettinger
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1465406&group_id=5470

Please note that this message will contain a full copy of the comment thread,
including the initial issue submission, for this request,
not just the latest update.
Category: Parser/Compiler
Group: None
>Status: Closed
>Resolution: Rejected
Priority: 5
Submitted By: Chris Wilson (ciw42)
Assigned to: Nobody/Anonymous (nobody)
Summary: Allowing the definition of constants

Initial Comment:
One of the current optimizations due in v2.5 includes
constant folding of expressions, which as it stands
serves as a way of somply getting rid of a redundant
arithmetic operations and the like.

In practice, it's rare a developer would leave an
expression such as "2+3" sat in his/her code, but by
allowing the declaration of constants within a script,
it could make this new feature *much* more useful.

As an example, in a recent script I had the following
at the top, outside the main loop:

SCREEN_WIDTH=640
SCREEN_HEIGHT=480
SCREEN_RATIO=SCREEN_WIDTH/SCREEN_HEIGHT

As SCREEN_RATIO is used a number of times during my
main loop, it makes sense to pre-calculate it to avoid
the extra processing, but if the compiler were aware
that SCREEN_WIDTH and SCREEN_HEIGHT were constants, it
could optimise out the calculation and I could include
the calculation in-place.

I frequently make use of "constants" to make my code
more readable, and wonder whether there is any
performance penalty or lack of optimisation going on
due to them in fact being regular variables?

----------------------------------------------------------------------

>Comment By: Raymond Hettinger (rhettinger)
Date: 2006-04-05 18:57

Message:
Logged In: YES 
user_id=80475

Python is too dynamic for this kind of optimization to be 
done automatically.  If those "constants" are defined at 
the module level, they can be changed by code outside the 
module.  Even within the module, it would take a thorough 
analysis of the code to determine that nothing was trying 
to alter the value of the global variable.  If 
the "constant" is defined inside a function, it is still a 
local variable subject to change by later lines in 
function.

Your best bet is to use the bind_consts recipe at ASPN.  
It will automatically turn some global references into 
locals:  
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/277
940

It may be possible to adapt the recipe to go an additional 
step and fold the globals "constants".

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=355470&aid=1465406&group_id=5470