having both dynamic and static variables

[Westley Martínez]

On Sat, 2011-03-05 at 18:37 -0800, John Nagle wrote:
> On 3/2/2011 9:27 PM, Steven D'Aprano wrote:
> > On Wed, 02 Mar 2011 19:45:16 -0800, Yingjie Lan wrote:
> >
> >> Hi everyone,
> >>
> >> Variables in Python are resolved dynamically at runtime, which comes at
> >> a performance cost. However, a lot of times we don't need that feature.
> >> Variables can be determined at compile time, which should boost up
> >> speed.
> > [...]
> >
> > This is a very promising approach taken by a number of projects.
> 
>     It's worth having some syntax for constants.  I'd suggest
> using "let":
> 
> 	let PI = 3.1415926535897932384626433832795028841971693993751
> 
> I'd propose the following semantics:
> 
> 1.  "let" creates an object whose binding is unchangeable.  This
>      is effectively a constant, provided that the value is immutable.
>      A compiler may treat such variables as constants for optimization
>      purposes.
> 
> 2.  Assignment to a a variable created with "let" produces an error
>      at compile time or run time.
> 
> 3.  Names bound with "let" have the same scope as any other name
>      created in the same context.  Function-local "let" variables
>      are permitted.
> 
> 4.  It is an error to use "let" on a name explicitly made "global",
>      because that would allow access to the variable before it was
>      initialized.
> 
> This is close to the semantics of "const" in C/C++, except that
> there's no notion of a const parameter.
> 
> "let" allows the usual optimizations - constant folding, hoisting
> out of loops, compile time arithmetic, unboxing, etc.  Ordinarily,
> Python compilers have to assume that any variable can be changed
> at any time from another thread, requiring worst-case code for
> everything.
> 
> 				John Nagle	
I'm against constants, for the purpose of "programmers should be smart
enough to not set a variable to another value that should be static",
but if Python were to have constants I think it would be better to use
something more descriptive than 'let'. Also, because the defined
constant is static, I think it would be better to use 'is' instead of
'='. Example:

constant x is 5