problem with exec and locals()

[Uwe Schmitt]

On 1 Jul., 15:15, Mel <mwil... at the-wire.com> wrote:
> rocksportrockerwrote:
>
> > Hi,
>
> > the following code does not work until I ommit the "a=0" statement.
>
> >    def test():
> >        exec "a=3" in locals()
> >        print a
> >        a=0
>
> >     test()
>
> > print raises:
> >      UnboundLocalError: local variable 'a' referenced before
> > assignment
>
> > Can anybody explain what is going wrong here ?
>
> AFAIK, local variables are implemented rather like __slots__ in new-style
> classes.  This is a very valuable efficiency measure, but it can cause this
> kind of trouble.  Without `a=0`, the bytecode compiler makes no slot for a,
> and dis.dis shows the following bytecode for test:>>> dis.dis (test)
>
>   2           0 LOAD_CONST               1 ('a=3')
>               3 LOAD_NAME                0 (locals)
>               6 CALL_FUNCTION            0
>               9 DUP_TOP
>              10 EXEC_STMT
>
>   3          11 LOAD_NAME                1 (a)
>              14 PRINT_ITEM
>              15 PRINT_NEWLINE
>              16 LOAD_CONST               0 (None)
>              19 RETURN_VALUE
>
> At address 11, LOAD_NAME 1(a) gets the value that was set by exec.
>
> With a=0, the code is>>> dis.dis(test2)
>
>   2           0 LOAD_CONST               1 ('a=4')
>               3 LOAD_NAME                0 (locals)
>               6 CALL_FUNCTION            0
>               9 DUP_TOP
>              10 EXEC_STMT
>
>   3          11 LOAD_FAST                0 (a)
>              14 PRINT_ITEM
>              15 PRINT_NEWLINE
>
>   4          16 LOAD_CONST               2 (0)
>              19 STORE_FAST               0 (a)
>              22 LOAD_CONST               0 (None)
>              25 RETURN_VALUE
>
> and here, the value of a is found in slot 0 via LOAD_FAST.  Slot 0 is used
> because a=0 forced a to be a local variable.
>
> Apparently, exec in locals() knows nothing about slots (because locals() is
> the only dictionary in the universe where slots would be involved ? --
> perhaps not, but close).
>
>         Mel.

Thanks for your answer. I wonder if this is a bug, or did I miss
something
in the docs ???

Greetings, Uwe