Using const() in different classes

[fmafma]

I found a strange behaviour, probably due to my lack of const() understanding.

If the test.py module contains:

class A:
    C = const(1)

class B:
    C = const(2)

and I try to import this module, I get:

>>> import test
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/lib/test.py", line 4, in B
SyntaxError: can't assign to expression

What I did wrong?

[fmafma]

Also, if I type this:

>>> CONST_X = const(123)
>>> CONST_Y = const(2 * CONST_X + 1)

I get:

Traceback (most recent call last):
  File "<stdin>", line 1
SyntaxError: not a constant

But if I put these 2 lines in a module and import that module, it works!

[robert-hh]

When you assign a value to a symbol using const() the compiler treats that symbol as a constant value, just like a number. These symbols are global in a script, not local to a class to function. So the second assignment it treated by the compiler like 1 = const(2).

[robert-hh]

In REPL compiler constants are not memorized between command lines. Each line is a separate call of the compiler. So in the second line the value of CONST_X is lost.

[fmafma]

Ok, I understand for the first example. So const() is more like creating an alias/define? (the documentation is not very clear about that)

But I don't understand your second answer:

>>> A = const(1)
>>> 1

So it still exists between the 2 calls. So why it can't use it in the next const() definition?

[robert-hh]

Seems you#re right. Then it may be that const symbols must not be used in an expressing when using REPL.

[fmafma]

Mmm, yes and no:

>>> a = const(1)
>>> b = a + 1
>>> c = sqrt(a + 1)
>>> d = const(a + 1)
Traceback (most recent call last):
  File "<stdin>", line 1
SyntaxError: not a constant

It only fails when used in const()...

[fmafma]

Another strange thing:

>>> const(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'const' isn't defined
>>> a = const(2)
>>>

I think I will stay away from const() for now ;o)

[peterhinch]

const is a directive to the Python compiler and doesn't really make sense at the REPL. If in a module you have

_WIDTH = const(256)

and later in your code you have

def foo(x):
    y  = x + _WIDTH

the bytecode compiler use the number 256 instead of looking up a global variable. Saves time and code.

[fmafma]

Ok, but why does it have to become global? Why not follow the Python object hierarchy?
As I said, I see it more as a define... I find confusing the way it is implemented.