gh-122300: Preserve AST nodes for format specifiers with single elements (#122308)

Author: pablogsal

Doc/library/ast.rst

@@ -316,7 +316,9 @@ Literals
                             args=[
                                 Name(id='a', ctx=Load())]),
                         conversion=-1,
-                        format_spec=Constant(value='.3'))]))
+                        format_spec=JoinedStr(
+                            values=[
+                                Constant(value='.3')]))]))
 
 
 .. class:: List(elts, ctx)

Lib/test/test_ast.py

@@ -3638,7 +3638,7 @@ def main():
 ('Expression', ('Subscript', (1, 0, 1, 10), ('List', (1, 0, 1, 3), [('Constant', (1, 1, 1, 2), 5, None)], ('Load',)), ('Slice', (1, 4, 1, 9), ('Constant', (1, 4, 1, 5), 1, None), ('Constant', (1, 6, 1, 7), 1, None), ('Constant', (1, 8, 1, 9), 1, None)), ('Load',))),
 ('Expression', ('IfExp', (1, 0, 1, 21), ('Name', (1, 9, 1, 10), 'x', ('Load',)), ('Call', (1, 0, 1, 5), ('Name', (1, 0, 1, 3), 'foo', ('Load',)), [], []), ('Call', (1, 16, 1, 21), ('Name', (1, 16, 1, 19), 'bar', ('Load',)), [], []))),
 ('Expression', ('JoinedStr', (1, 0, 1, 6), [('FormattedValue', (1, 2, 1, 5), ('Name', (1, 3, 1, 4), 'a', ('Load',)), -1, None)])),
-('Expression', ('JoinedStr', (1, 0, 1, 10), [('FormattedValue', (1, 2, 1, 9), ('Name', (1, 3, 1, 4), 'a', ('Load',)), -1, ('Constant', (1, 5, 1, 8), '.2f', None))])),
+('Expression', ('JoinedStr', (1, 0, 1, 10), [('FormattedValue', (1, 2, 1, 9), ('Name', (1, 3, 1, 4), 'a', ('Load',)), -1, ('JoinedStr', (1, 4, 1, 8), [('Constant', (1, 5, 1, 8), '.2f', None)]))])),
 ('Expression', ('JoinedStr', (1, 0, 1, 8), [('FormattedValue', (1, 2, 1, 7), ('Name', (1, 3, 1, 4), 'a', ('Load',)), 114, None)])),
 ('Expression', ('JoinedStr', (1, 0, 1, 11), [('Constant', (1, 2, 1, 6), 'foo(', None), ('FormattedValue', (1, 6, 1, 9), ('Name', (1, 7, 1, 8), 'a', ('Load',)), -1, None), ('Constant', (1, 9, 1, 10), ')', None)])),
 ]

Misc/NEWS.d/next/Core and Builtins/2024-07-26-14-05-51.gh-issue-122300.SVIF-l.rst

@@ -0,0 +1,2 @@
+Preserve AST nodes for f-string with single-element format specifiers. Patch
+by Pablo Galindo

Parser/action_helpers.c

@@ -1010,7 +1010,8 @@ _PyPegen_setup_full_format_spec(Parser *p, Token *colon, asdl_expr_seq *spec, in
         spec = resized_spec;
     }
     expr_ty res;
-    if (asdl_seq_LEN(spec) == 0) {
+    Py_ssize_t n = asdl_seq_LEN(spec);
+    if (n == 0 || (n == 1 && asdl_seq_GET(spec, 0)->kind == Constant_kind)) {
         res = _PyAST_JoinedStr(spec, lineno, col_offset, end_lineno,
                                     end_col_offset, p->arena);
     } else {