[Python-Dev] Those import related syntax errors again...

[Jeremy Hylton]

>>>>> "SP" == Samuele Pedroni <pedroni@inf.ethz.ch> writes:

  SP> I should reformulate: I think a possible not arbitrary rule for
  SP> exec is only exec ... in ... is valid, but this also something
  SP> ok only on the long-run (like import * deprecation).

Yes.

  SP> Then it is necessary to agree on the semantic of locals().

That's easy.  Make the warning in the current documentation a feature:
locals() returns a dictionary representing the local symbol table.
The effects of modifications to this dictionary is undefined.

  SP> What would happen right now mixing lexical scoping and exec
  SP> ... in locals()?

Right now, the exec would get flagged as an error.  If it were allowed
to execute, the exec would operator on the frame's f_locals dict.  The
locals() builtin calls the following function.

PyObject *
PyEval_GetLocals(void)
{
	PyFrameObject *current_frame = PyThreadState_Get()->frame;
	if (current_frame == NULL)
		return NULL;
	PyFrame_FastToLocals(current_frame);
	return current_frame->f_locals;
}

This copies all variables from the fast slots into the f_locals
dictionary.  When the exec statement is executed, it does the reverse
copying from the locals dict back into the fast slots.

The FastToLocals and LocalsToFast functions don't know anything about
the closure, so those variables simply wouldn't affected.  Assignments
in the exec would be ignored by nested scopes.

Jeremy