The execfile() docs imply it acts on locals same as exec. But in truth

it acts more like assigning to keys in locals(), i.e. modifications to
function locals aren't reflected in the locals when execfile() returns.

Doc/lib/libfuncs.tex

@@ -257,6 +257,13 @@ class instances are callable if they have a \method{__call__()} method.
   If both dictionaries are omitted, the expression is executed in the
   environment where \function{execfile()} is called.  The return value is
   \code{None}.
+
+  \strong{Warning:} The default \var{locals} act as described for function
+  \function{locals()} below:  modifications to the default \var{locals}
+  dictionary should not be attempted.  Pass an explicit \var{locals}
+  dictionary if you need to see effects of the code on \var{locals} after
+  function \function{execfile()} returns.  \function{execfile()} cannot
+  be used reliably to modify a function's locals.
 \end{funcdesc}
 
 \begin{funcdesc}{file}{filename\optional{, mode\optional{, bufsize}}}