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bpo-38870: Only omit slice parantheses when the inner expression is simple #20152

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Merged
pablogsal merged 3 commits into from
May 18, 2020
Merged

bpo-38870: Only omit slice parantheses when the inner expression is simple #20152

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45f7ffd
bpo-38870: Only ignore parens when the inside expression is simple
isidentical May 17, 2020
6d45f39
extract to the function
isidentical May 18, 2020
f5b3508
tell why instead of how in the comments
isidentical May 18, 2020
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12 changes: 11 additions & 1 deletion Lib/ast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1346,10 +1346,20 @@ def visit_Call(self, node):
self.traverse(e)

def visit_Subscript(self, node):
def is_simple_tuple(slice_value):
# when unparsing a non-empty tuple, the parantheses can be safely
# omitted if there aren't any elements that explicitly requires
# parantheses (such as starred expressions).
return (
isinstance(slice_value, Tuple)
and slice_value.elts
and not any(isinstance(elt, Starred) for elt in slice_value.elts)
)

self.set_precedence(_Precedence.ATOM, node.value)
self.traverse(node.value)
with self.delimit("[", "]"):
if isinstance(node.slice, Tuple) and node.slice.elts:
if is_simple_tuple(node.slice):
self.items_view(self.traverse, node.slice.elts)
else:
self.traverse(node.slice)
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10 changes: 9 additions & 1 deletion Lib/test/test_unparse.py
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Expand Up @@ -279,10 +279,13 @@ def test_dict_unpacking_in_dict(self):
self.check_ast_roundtrip(r"""{**{'y': 2}, 'x': 1}""")
self.check_ast_roundtrip(r"""{**{'y': 2}, **{'x': 1}}""")

def test_ext_slices(self):
def test_slices(self):
self.check_ast_roundtrip("a[i]")
self.check_ast_roundtrip("a[i,]")
self.check_ast_roundtrip("a[i, j]")
self.check_ast_roundtrip("a[(*a,)]")
self.check_ast_roundtrip("a[(a:=b)]")
self.check_ast_roundtrip("a[(a:=b,c)]")
self.check_ast_roundtrip("a[()]")
self.check_ast_roundtrip("a[i:j]")
self.check_ast_roundtrip("a[:j]")
Expand Down Expand Up @@ -466,6 +469,11 @@ def test_unary_op_factor(self):
for prefix in ("not",):
self.check_src_roundtrip(f"{prefix} 1")

def test_slices(self):
self.check_src_roundtrip("a[1]")
self.check_src_roundtrip("a[1, 2]")
self.check_src_roundtrip("a[(1, *a)]")

class DirectoryTestCase(ASTTestCase):
"""Test roundtrip behaviour on all files in Lib and Lib/test."""

Expand Down