Note that it is illegal to delete a cell variable.

jeremyhylton · jeremyhylton · commit d09ed68aeb01 · 2002-04-01T21:15:14.000Z
Note that deleteing an unbound local will raise a NameError.
diff --git a/Doc/ref/ref6.tex b/Doc/ref/ref6.tex
@@ -338,12 +338,16 @@ \section{The \keyword{del} statement \label{del}}
 Deletion of a target list recursively deletes each target, from left
 to right.
 
-Deletion of a name removes the binding of that name (which must exist)
+Deletion of a name removes the binding of that name 
 from the local or global namespace, depending on whether the name
-occurs in a \keyword{global} statement in the same code block.
+occurs in a \keyword{global} statement in the same code block.  If the
+name is unbound, a \exception{NameError} exception will be raised.
 \stindex{global}
 \indexii{unbinding}{name}
 
+It is illegal to delete a name from the local namespace if it occurs
+as a free variable\indexii{free}{varaible} in a nested block.
+
 Deletion of attribute references, subscriptions and slicings
 is passed to the primary object involved; deletion of a slicing
 is in general equivalent to assignment of an empty slice of the
