[Python-Dev] PEP 3000 and exec

[skip@pobox.com]

    >> This might be minor-- but I didn't see anyone mentioning it so far.
    >> If `exec` functionality is to be provided, then I think it still
    >> should be a keyword for the parser to know; currently bytecode
    >> generation is affected if `exec` is present.  Even if that changes
    >> for Python 3k (we don't know yet), the paragraph for exec should be
    >> annotated with a note about this issue.

    Brett> But the PEP says that 'exec' will become a function and thus no
    Brett> longer become a built-in, so changing the grammar is not needed.

I don't think that was the OP's point though it might not have been terribly
clear.  Today, the presence of the exec statement in a function changes how
non-local load instructions are generated.  Consider f and g with their
dis.dis output:

    >>> def f(a):
    ...   exec "import %s" % a
    ...   print q
    ... 
    >>> def g(a):
    ...   __import__(a)
    ...   print q
    ... 
    >>> dis.dis(f)
      2           0 LOAD_CONST               1 ('import %s')
                  3 LOAD_FAST                0 (a)
                  6 BINARY_MODULO       
                  7 LOAD_CONST               0 (None)
                 10 DUP_TOP             
                 11 EXEC_STMT           

      3          12 LOAD_NAME                1 (q)
                 15 PRINT_ITEM          
                 16 PRINT_NEWLINE       
                 17 LOAD_CONST               0 (None)
                 20 RETURN_VALUE        
    >>> dis.dis(g)
      2           0 LOAD_GLOBAL              0 (__import__)
                  3 LOAD_FAST                0 (a)
                  6 CALL_FUNCTION            1
                  9 POP_TOP             

      3          10 LOAD_GLOBAL              2 (q)
                 13 PRINT_ITEM          
                 14 PRINT_NEWLINE       
                 15 LOAD_CONST               0 (None)
                 18 RETURN_VALUE        

If the exec statement is replaced by a function, how will the bytecode
generator know that q should be looked up using LOAD_NAME instead of
LOAD_GLOBAL?  Maybe it's a non-issue, but even if so, a note to that affect
on the wiki page might be worthwhile.

Skip