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

Guido van Rossum guido at python.org
Tue Oct 11 00:05:56 CEST 2005

My idea was to make the compiler smarter so that it would recognize exec() even if it was just a function.

Another idea might be to change the exec() spec so that you are required to pass in a namespace (and you can't use locals() either!). Then the whole point becomes moot.

On 10/10/05, skip at pobox.com <skip at pobox.com> wrote:

>> 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 LOADCONST 1 ('import %s') 3 LOADFAST 0 (a) 6 BINARYMODULO 7 LOADCONST 0 (None) 10 DUPTOP 11 EXECSTMT 3 12 LOADNAME 1 (q) 15 PRINTITEM 16 PRINTNEWLINE 17 LOADCONST 0 (None) 20 RETURNVALUE >>> dis.dis(g) 2 0 LOADGLOBAL 0 (import) 3 LOADFAST 0 (a) 6 CALLFUNCTION 1 9 POPTOP 3 10 LOADGLOBAL 2 (q) 13 PRINTITEM 14 PRINTNEWLINE 15 LOADCONST 0 (None) 18 RETURNVALUE If the exec statement is replaced by a function, how will the bytecode generator know that q should be looked up using LOADNAME instead of LOADGLOBAL? 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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