bpo-10544: Deprecate "yield" in comprehensions and generator expressions. (GH-4579)

The current behaviour of yield expressions inside comprehensions  and
generator expressions is essentially an accident of implementation - it
arises implicitly from the way the compiler handles yield expressions inside
nested functions and generators.

Since the current behaviour wasn't deliberately designed, and is inherently
confusing, we're deprecating it, with no current plans to reintroduce it.
Instead, our advice will be to use a named nested generator definition
for cases where this behaviour is desired.

1 files changed, 30 insertions, 1 deletions

diff --git a/Python/symtable.c b/Python/symtable.c
index 55815c91cc..bbac25cf37 100644
--- a/Python/symtable.c
+++ b/Python/symtable.c
@@ -1734,7 +1734,6 @@ symtable_handle_comprehension(struct symtable *st, expr_ty e,
                               e->lineno, e->col_offset)) {
         return 0;
     }
-    st->st_cur->ste_generator = is_generator;
     if (outermost->is_async) {
         st->st_cur->ste_coroutine = 1;
     }
@@ -1754,6 +1753,36 @@ symtable_handle_comprehension(struct symtable *st, expr_ty e,
     if (value)
         VISIT(st, expr, value);
     VISIT(st, expr, elt);
+    if (st->st_cur->ste_generator) {
+        PyObject *msg = PyUnicode_FromString(
+            (e->kind == ListComp_kind) ? "'yield' inside list comprehension" :
+            (e->kind == SetComp_kind) ? "'yield' inside set comprehension" :
+            (e->kind == DictComp_kind) ? "'yield' inside dict comprehension" :
+            "'yield' inside generator expression");
+        if (msg == NULL) {
+            symtable_exit_block(st, (void *)e);
+            return 0;
+        }
+        if (PyErr_WarnExplicitObject(PyExc_DeprecationWarning,
+                msg, st->st_filename, st->st_cur->ste_lineno,
+                NULL, NULL) == -1)
+        {
+            if (PyErr_ExceptionMatches(PyExc_DeprecationWarning)) {
+                /* Replace the DeprecationWarning exception with a SyntaxError
+                   to get a more accurate error report */
+                PyErr_Clear();
+                PyErr_SetObject(PyExc_SyntaxError, msg);
+                PyErr_SyntaxLocationObject(st->st_filename,
+                                           st->st_cur->ste_lineno,
+                                           st->st_cur->ste_col_offset);
+            }
+            Py_DECREF(msg);
+            symtable_exit_block(st, (void *)e);
+            return 0;
+        }
+        Py_DECREF(msg);
+    }
+    st->st_cur->ste_generator |= is_generator;
     return symtable_exit_block(st, (void *)e);
 }