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Transformer_NonRecursive in lark-parser/lark

The author described this change as “Transformer_NonRecursive should respond as documented to a Discard exception being raised”. It counts as a record because the check below fails on the code as it stood at 1f56497de and passes on 50e789ab9, with nothing else changed between the two runs.

Projectlark-parser/lark
Fix saved2021-10-04
Sharing licenceMIT · LICENSE
Change size+8 −2

What the code was meant to do, written into the code itself as a docstring

Same as Transformer but non-recursive. Like Transformer, it doesn't change the original tree. Useful for huge trees.

The change

2626 del stack[-size:]
2727 else:
2828 args = []
29- stack.append(self._call_userfunc(x, args))
29+ try:
30+ stack.append(self._call_userfunc(x, args))
31+ except Discard:
32+ pass
3033 elif self.__visit_tokens__ and isinstance(x, Token):
31- stack.append(self._call_userfunc_token(x))
34+ try:
35+ stack.append(self._call_userfunc_token(x))
36+ except Discard:
37+ pass
3238 else:
3339 stack.append(x)
3440

The check that tells the two apart

fail→pass·tests/test_trees.py::TestTrees::test_transformer_variants

Check file tests/test_trees.py, taken without changes from the fix and copied onto the older code, so the exact same check runs against both versions.

Origin and history

The fix50e789ab9d225cb2a5a07d4246473578f43cdf3d
The code before it1f56497de8d90c6136dcda0ce71f4de3b298a07e
Broken version dated2021-09-24
Modulelark.visitors
Units changedTransformer_NonRecursive
Fingerprint6a8a9b799ac862e3
Checked2026-08-18 by goldset/0.1

Every field above is generated by our program. None of it is written by hand.

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