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pythongh-106396: Special-case empty format spec to gen empty JoinedSt…
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…r node (python#106401)
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lysnikolaou authored Jul 4, 2023
1 parent 8f6df5e commit dfe4de2
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18 changes: 18 additions & 0 deletions Lib/test/test_fstring.py
Original file line number Diff line number Diff line change
Expand Up @@ -496,6 +496,24 @@ def test_ast_line_numbers_with_parentheses(self):
self.assertEqual(wat2.end_col_offset, 17)
self.assertEqual(fstring.end_col_offset, 18)

def test_ast_fstring_empty_format_spec(self):
expr = "f'{expr:}'"

mod = ast.parse(expr)
self.assertEqual(type(mod), ast.Module)
self.assertEqual(len(mod.body), 1)

fstring = mod.body[0].value
self.assertEqual(type(fstring), ast.JoinedStr)
self.assertEqual(len(fstring.values), 1)

fv = fstring.values[0]
self.assertEqual(type(fv), ast.FormattedValue)

format_spec = fv.format_spec
self.assertEqual(type(format_spec), ast.JoinedStr)
self.assertEqual(len(format_spec.values), 0)

def test_docstring(self):
def f():
f'''Not a docstring'''
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
When the format specification of an f-string expression is empty, the parser now
generates an empty :class:`ast.JoinedStr` node for it instead of an one-element
:class:`ast.JoinedStr` with an empty string :class:`ast.Constant`.
12 changes: 12 additions & 0 deletions Parser/action_helpers.c
Original file line number Diff line number Diff line change
Expand Up @@ -997,6 +997,18 @@ _PyPegen_setup_full_format_spec(Parser *p, Token *colon, asdl_expr_seq *spec, in
if (!spec) {
return NULL;
}

// This is needed to keep compatibility with 3.11, where an empty format spec is parsed
// as an *empty* JoinedStr node, instead of having an empty constant in it.
if (asdl_seq_LEN(spec) == 1) {
expr_ty e = asdl_seq_GET(spec, 0);
if (e->kind == Constant_kind
&& PyUnicode_Check(e->v.Constant.value)
&& PyUnicode_GetLength(e->v.Constant.value) == 0) {
spec = _Py_asdl_expr_seq_new(0, arena);
}
}

expr_ty res = _PyAST_JoinedStr(spec, lineno, col_offset, end_lineno, end_col_offset, p->arena);
if (!res) {
return NULL;
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