Local annotation turns local variables in cells

[markshannon]

Bug report

Bug description:

Compiling this function

def f(x):
    a:x
    return x

Gives this bytecode:

  --           MAKE_CELL                0 (x)

   1           RESUME                   0

   3           LOAD_DEREF               0 (x)
               RETURN_VALUE

Local variable annotations are supposed to be ignored by Python (they are used by static type checkers only),
so the addition of the annotation should not change the generated code.
The expected disassembly:

   1           RESUME                   0

   3           LOAD_LAST                0 (x)
               RETURN_VALUE

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-130924: Do not create cells for usages of names in local annotations #131843

[gvanrossum]

Maybe @JelleZijlstra can look into this -- it smells like an unanticipated effect of PEP 749?

[JelleZijlstra]

Yes, I already assigned it to myself to look into.

I think this is harmless in the sense that it doesn't change any behavior we should guarantee, but it's a pessimization.

[sergey-miryanov]

I can confirm that this is related to PEP-749.
a:x introduces a child scope for __annotate__ and x is a free variable for that scope and this is poisons x for the whole function.