Should constants be introduced to Python?

[bartc]

On 16/11/2017 06:16, Saeed Baig wrote:
> Hey guys I am thinking of perhaps writing a PEP to introduce constants to Python. Something along the lines of Swift’s “let” syntax (e.g. “let pi = 3.14”).
> 
> Since I’m sort of new to this, I just wanted to ask:
> - Has a PEP for this already been written? If so, where can I find the link/info to it?
> - Do you guys think it would be a good idea? Why or why not? Do you think there’s a better way to do it? I’d like to know what others think about this idea before making any formal submission.
> 


I've taken part in a few discussions here on the subject.

However, if you're going to write a PEP, does that require that you have 
some idea of how it would be implemented?

For example, you write this:

   let A = 10
   let B = 20
   x = A + B

On CPython, would the byte-code compiler reduce the A+B to 30, or would 
it still do the calculation?

Suppose those lets were in an imported module M:

   import M

   x = M.A + M.B

Would it reduce the expression here or not? (And if so, how? Remember 
that M itself is not constant, so M could have been reassigned as 
another module, or a class, so that another M.A is not a constant.)

What would happen here:

   let A = 10
   A = 12

Would the assignment be allowed or not? Because the way Python works 
now, is that EVERY (top-level) identifier created by the user is a 
variable, and can be assigned something else. To prohibit A=12 would 
mean creating a new category of identifier that is fixed.

And the error would be hard to pick up at compile-time here:

   M.A = 12

As the category of M.A is not known to the byte-code compiler (also that 
M is not constant as I said).

Note that names are created also with 'import', 'def' and 'class', and 
it might be desirable to have these constant too, for example:

    let def F():

-- 
bartc