gh-125331: Allow the parser to activate future imports on the fly

Author: pablogsal

Grammar/python.gram

@@ -207,7 +207,9 @@ import_name[stmt_ty]: 'import' a=dotted_as_names { _PyAST_Import(a, EXTRA) }
 # note below: the ('.' | '...') is necessary because '...' is tokenized as ELLIPSIS
 import_from[stmt_ty]:
     | 'from' a=('.' | '...')* b=dotted_name 'import' c=import_from_targets {
-        _PyAST_ImportFrom(b->v.Name.id, c, _PyPegen_seq_count_dots(a), EXTRA) }
+        _PyPegen_check_future_import(p,
+            _PyAST_ImportFrom(b->v.Name.id, c, _PyPegen_seq_count_dots(a), EXTRA)
+        ) }
     | 'from' a=('.' | '...')+ 'import' b=import_from_targets {
         _PyAST_ImportFrom(NULL, b, _PyPegen_seq_count_dots(a), EXTRA) }
 import_from_targets[asdl_alias_seq*]:

Parser/action_helpers.c

@@ -1691,3 +1691,24 @@ _PyPegen_concatenate_strings(Parser *p, asdl_expr_seq *strings,
     assert(current_pos == n_elements);
     return _PyAST_JoinedStr(values, lineno, col_offset, end_lineno, end_col_offset, p->arena);
 }
+
+stmt_ty
+_PyPegen_check_future_import(Parser *p, stmt_ty importfrom) {
+    if (importfrom->kind != ImportFrom_kind) {
+        goto exit;
+    }
+    asdl_alias_seq *names = importfrom->v.ImportFrom.names;
+    if (asdl_seq_LEN(names) != 1) {
+        goto exit;
+    }
+    identifier mod = importfrom->v.ImportFrom.module;
+    if (PyUnicode_CompareWithASCIIString(mod, "__future__") != 0) {
+        goto exit;
+    }
+    alias_ty alias = asdl_seq_GET(names, 0);
+    if (PyUnicode_CompareWithASCIIString(alias->name, "barry_as_FLUFL") == 0) {
+        p->flags |= PyPARSE_BARRY_AS_BDFL;
+    }
+exit:
+    return importfrom;
+}

Parser/parser.c

@@ -361,6 +361,8 @@ asdl_stmt_seq *_PyPegen_interactive_exit(Parser *);
 // TODO: move to the correct place in this file
 expr_ty _PyPegen_joined_str(Parser *p, Token* a, asdl_expr_seq* expr, Token*b);
 
+stmt_ty _PyPegen_check_future_import(Parser *p, stmt_ty importfrom);
+
 // Generated function in parse.c - function definition in python.gram
 void *_PyPegen_parse(Parser *);