const() function

[pythoncoder]

Thanks Damian for that clear explanation. Can const() effectively be applied to class variables? It clarifies code to define constants relevant to a single class as class variables.

[Damien]

Can const() effectively be applied to class variables? It clarifies code to define constants relevant to a single class as class variables.

Yes, const works for class variables and function local variables (the latter could do with further optimisation).

[SpotlightKid]

I noticed one thing that doesn't work with const():

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NAME = ALIAS = const(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'const' is not defined

Which should be perfectly valid Python syntax. Is this intended?

[pfalcon]

Is this intended?

Consider yes. Consider "const" as a special feature which works only for syntax of the form "NAME = const(<scalar_value>)".

[SpotlightKid]

So what if I do:

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NAME = const(42)
ALIAS = NAME

Is ALIAS also considered a constant then?

[pfalcon]

No. Only object initialized with a const() function is consider a constant. For ALIAS to be a constant it would need to be:

ALIAS = const(NAME)

And that may work or not as a constant underlyingly. Generally, const() is a hint to MicroPython compiler to consider constant propagation. It's semantics is equivalent to:

const = lambda x: x

(That's also how it should be defined for CPython compatibility).

[badi]

Regarding "using const for class variable":
I define following class

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class foo:
    stVar = const(42)
    stVar1=24
    def __init__(self):
        print(self.stVar)
        print(self.stVar1)
        print(foo.stVar)
        print(foo.stVar1)

which gives me:

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>>> a =foo()
<class 'foo'>
24
<class 'type'>
24

Why?

[dhylands]

I have have a feeling that this is related to this issue: https://github.com/micropython/micropython/issues/1675