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gh-143055: Fix crash in AST unparser when unparsing dict comprehension unpacking #145556

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gh-143055: Fix crash in AST unparser when unparsing dict comprehension unpacking #145556

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7fdeed5
Fix crash in AST unparser when unparsing dict comprehension unpacking
StanFromIreland Mar 5, 2026
78adfb8
I was overthinking it.
StanFromIreland Mar 5, 2026
727721a
test list comp too
StanFromIreland Mar 5, 2026
e3d8248
Merge branch 'main' into dict-comp-unparse
StanFromIreland Mar 5, 2026
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1 change: 1 addition & 0 deletions Doc/conf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -556,6 +556,7 @@
# mapping unique short aliases to a base URL and a prefix.
# https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html
extlinks = {
"oss-fuzz": ("https://issues.oss-fuzz.com/issues/%s", "#%s"),
"pypi": ("https://pypi.org/project/%s/", "%s"),
"source": (SOURCE_URI, "%s"),
}
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10 changes: 7 additions & 3 deletions Lib/_ast_unparse.py
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Expand Up @@ -738,9 +738,13 @@ def visit_SetComp(self, node):

def visit_DictComp(self, node):
with self.delimit("{", "}"):
self.traverse(node.key)
self.write(": ")
self.traverse(node.value)
if node.value:
self.traverse(node.key)
self.write(": ")
self.traverse(node.value)
else:
self.write("**")
self.traverse(node.key)
for gen in node.generators:
self.traverse(gen)

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2 changes: 2 additions & 0 deletions Lib/test/test_future_stmt/test_future.py
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Expand Up @@ -349,6 +349,8 @@ def test_annotations(self):
eq("(i ** 2 + j for i in (1, 2, 3) for j in (1, 2, 3))")
eq("{i: 0 for i in (1, 2, 3)}")
eq("{i: j for i, j in ((1, 'a'), (2, 'b'), (3, 'c'))}")
eq("{**x for x in ()}")
eq("[*x for x in ()]")
eq("[(x, y) for x, y in (a, b)]")
eq("[(x,) for x, in (a,)]")
eq("Python3 > Python2 > COBOL")
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@JelleZijlstra JelleZijlstra Mar 5, 2026

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Would you mind adding tests for the corresponding case with list comprehensions too?

>>> from __future__ import annotations
>>> def f(x: [*x for x in y]): pass
... 
>>> f.__annotations__
{'x': '[*x for x in y]'}

It seems to work correctly already but we should test it explicitly.

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4 changes: 4 additions & 0 deletions Lib/test/test_unparse.py
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Expand Up @@ -403,6 +403,10 @@ def test_set_comprehension(self):
def test_dict_comprehension(self):
self.check_ast_roundtrip("{x: x*x for x in range(10)}")

def test_dict_comprehension_unpacking(self):
self.check_ast_roundtrip("{**x for x in ()}")
self.check_ast_roundtrip("{**x for x in range(10)}")

def test_class_decorators(self):
self.check_ast_roundtrip(class_decorator)

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-05-16-16-17.gh-issue-143055.qDUFlY.rst
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@@ -0,0 +1,2 @@
Fix crash in AST unparser when unparsing dict comprehension unpacking.
Found by OSS Fuzz in :oss-fuzz:`489790200`.
12 changes: 9 additions & 3 deletions Python/ast_unparse.c
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Expand Up @@ -464,9 +464,15 @@ static int
append_ast_dictcomp(PyUnicodeWriter *writer, expr_ty e)
{
APPEND_CHAR('{');
APPEND_EXPR(e->v.DictComp.key, PR_TEST);
APPEND_STR(": ");
APPEND_EXPR(e->v.DictComp.value, PR_TEST);
if (e->v.DictComp.value) {
APPEND_EXPR(e->v.DictComp.key, PR_TEST);
APPEND_STR(": ");
APPEND_EXPR(e->v.DictComp.value, PR_TEST);
}
else {
APPEND_STR("**");
APPEND_EXPR(e->v.DictComp.key, PR_TEST);
}
APPEND(comprehensions, e->v.DictComp.generators);
APPEND_CHAR_FINISH('}');
}
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