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gh-137308: Replace a single docstring with pass #137318

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Aug 2, 2025
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gh-137308: Replace a single docstring with pass #137318

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gh-137308: Replace a single docstring with `pass`
sobolevn Aug 2, 2025
a3e9895
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sobolevn Aug 2, 2025
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23 changes: 23 additions & 0 deletions Lib/test/test_ast/test_ast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -220,6 +220,29 @@ def test_negative_locations_for_compile(self):
# This also must not crash:
ast.parse(tree, optimize=2)

def test_docstring_optimization_single_node(self):
# https://github.com/python/cpython/issues/137308
for code in [
'class A: """Docstring"""',
'def func(): """Docstring"""',
'async def func(): """Docstring"""',
]:
for opt_level in [0, 1, 2]:
with self.subTest(code=code, opt_level=opt_level):
mod = ast.parse(code, optimize=opt_level)
self.assertEqual(len(mod.body[0].body), 1)
if opt_level == 2:
self.assertIsInstance(mod.body[0].body[0], ast.Pass)
else:
self.assertIsInstance(mod.body[0].body[0], ast.Expr)
self.assertIsInstance(
mod.body[0].body[0].value,
ast.Constant,
)

compile(mod, "a", "exec")
compile(mod, "a", "exec", optimize=opt_level)

def test_slice(self):
slc = ast.parse("x[::]").body[0].value.slice
self.assertIsNone(slc.upper)
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-08-02-10-27-53.gh-issue-137308.at05p_.rst
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@@ -0,0 +1,3 @@
Replace a single docstring in a node body to a :keyword:`pass`, so node's
body is never empty. There was a :exc:`ValueError` in :func:`compile`
otherwise.
26 changes: 25 additions & 1 deletion Python/ast_preprocess.c
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Original file line number Diff line number Diff line change
Expand Up @@ -435,13 +435,37 @@ stmt_seq_remove_item(asdl_stmt_seq *stmts, Py_ssize_t idx)
return 1;
}

static int
remove_docstring(asdl_stmt_seq *stmts, Py_ssize_t idx, PyArena *ctx_)
{
// In case there's just the docstring in the body, replace it with `pass`
// keyword, so body won't be empty.
if (asdl_seq_LEN(stmts) == 1) {
stmt_ty docstring = (stmt_ty)asdl_seq_GET(stmts, 0);
stmt_ty pass = _PyAST_Pass(
docstring->lineno, docstring->col_offset,
// we know that `pass` always takes 4 chars and a single line,
// while docstring can span on multiple lines
docstring->lineno, docstring->col_offset + 4,
ctx_
);
if (pass == NULL) {
return 0;
}
asdl_seq_SET(stmts, 0, pass);
return 1;
}
// In case there are more than 1 body items, just remove the docstring.
return stmt_seq_remove_item(stmts, idx);
}

static int
astfold_body(asdl_stmt_seq *stmts, PyArena *ctx_, _PyASTPreprocessState *state)
{
int docstring = _PyAST_GetDocString(stmts) != NULL;
if (docstring && (state->optimize >= 2)) {
/* remove the docstring */
if (!stmt_seq_remove_item(stmts, 0)) {
if (!remove_docstring(stmts, 0, ctx_)) {
return 0;
}
docstring = 0;
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