having both dynamic and static variables

[MRAB]

On 06/03/2011 02:37, 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.
>
Having a fixed binding could be useful elsewhere, for example, with
function definitions:

     const PI = 3.1415926535897932384626433832795028841971693993751

     const def squared(x):
         return x * x

or:

     fixed PI = 3.1415926535897932384626433832795028841971693993751

     fixed def squared(x):
         return x * x