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bpo-41044: generate a valid python parser for opt+seq rules #20995

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Jun 20, 2020
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bpo-41044: generate a valid python parser for opt+seq rules #20995

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bpo-41044: generate a valid python parser for opt+seq rules
isidentical Jun 19, 2020
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isidentical Jun 20, 2020
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8 changes: 8 additions & 0 deletions Lib/test/test_peg_generator/test_pegen.py
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Original file line number Diff line number Diff line change
Expand Up @@ -493,6 +493,14 @@ def test_start_leader(self) -> None:
# Would assert False without a special case in compute_left_recursives().
make_parser(grammar)

def test_opt_sequence(self) -> None:
grammar = """
start: [NAME*]
"""
# This case was failing because of a double trailing comma at the end
# of a line in the generated source. See bpo-41044
make_parser(grammar)

def test_left_recursion_too_complex(self) -> None:
grammar = """
start: foo
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8 changes: 7 additions & 1 deletion Tools/peg_generator/pegen/python_generator.py
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Expand Up @@ -93,7 +93,13 @@ def visit_NegativeLookahead(self, node: NegativeLookahead) -> Tuple[None, str]:

def visit_Opt(self, node: Opt) -> Tuple[str, str]:
name, call = self.visit(node.node)
return "opt", f"{call}," # Note trailing comma!
# Note trailing comma (the call may already have one comma
# at the end, for example when rules have both repeat0 and optional
# markers, e.g: [rule*])
if call.endswith(","):
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Hummmm, not a find on relying on the generated code to reason about the grammar structure (I know that we do it in other places but we have been removing those as it makes it more difficult to reason about). In this case I have to say that is pretty straightforward so it may be fine.

return "opt", call
else:
return "opt", f"{call},"

def visit_Repeat0(self, node: Repeat0) -> Tuple[str, str]:
if node in self.cache:
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