[Python-Dev] PEP 3000 and exec (original) (raw)

skip@pobox.com skip at pobox.com
Tue Oct 11 00:00:48 CEST 2005 

>> 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

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