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Lark LALR calculator with a Transformer and error handling: working versions (tested 2026-10-16)

A small LALR arithmetic grammar parsed with lark and evaluated by a Transformer, plus how syntax errors and exceptions raised inside transformer methods surface.

passed5 passed · 0 failed2026-10-16 21:57 UTC
Status
passed
Last passed
2026-10-16 21:57 UTC
Tests
5 / 5
Language
Python
Runtime
Python 3.13.16
pinned versions2
lark@1.3.1pytest@9.1.1
run it
uv venv -q .venv && uv pip install -q --python .venv/bin/python -r requirements.txt
.venv/bin/python -m pytest -q
test environment
Python 3.13.16
linux 6.17.0-1022-azure
x64
runner: github-actions 38972656245

The code

Exactly the files that passed. Your agent can pull them with get_recipe("lark-lalr-calculator-transformer") (install), or see them on GitHub.

requirements.txt
lark==1.3.1
pytest==9.1.1
calculator.py
from lark import Lark, Transformer

GRAMMAR = r"""
?start: sum

?sum: product
    | sum "+" product -> add
    | sum "-" product -> sub

?product: atom
    | product "*" atom -> mul
    | product "/" atom -> div

?atom: NUMBER -> number
     | "-" atom   -> neg
     | "(" sum ")"

%import common.NUMBER
%import common.WS_INLINE
%ignore WS_INLINE
"""

def parse(text):
    return Lark(GRAMMAR, parser="lalr").parse(text)

class Calc(Transformer):
    def number(self, items):
        return float(items[0])

    def add(self, items):
        return items[0] + items[1]

    def sub(self, items):
        return items[0] - items[1]

    def mul(self, items):
        return items[0] * items[1]

    def div(self, items):
        return items[0] / items[1]

    def neg(self, items):
        return -items[0]

def evaluate(text):
    return Calc().transform(parse(text))
tests/test_calculator.py
import pytest
from lark.exceptions import (
    UnexpectedCharacters,
    UnexpectedInput,
    UnexpectedToken,
    VisitError,
)

from calculator import evaluate, parse

def test_precedence_and_parens():
    assert evaluate("2 + 3 * 4") == 14.0
    assert evaluate("-(2 + 3) * 4") == -20.0

def test_division_is_float():
    assert evaluate("7 / 2") == 3.5

def test_unexpected_token_reports_position_and_expected():
    with pytest.raises(UnexpectedToken) as exc:
        parse("1 + * 2")
    assert exc.value.line == 1 and exc.value.column == 5
    assert "NUMBER" in exc.value.expected

def test_unknown_character_is_not_a_token_error():
    with pytest.raises(UnexpectedCharacters) as exc:
        parse("1 # 2")
    assert (exc.value.line, exc.value.column) == (1, 3)
    assert isinstance(exc.value, UnexpectedInput)

def test_transformer_errors_are_wrapped_in_visit_error():
    with pytest.raises(VisitError) as exc:
        evaluate("1 / 0")
    assert isinstance(exc.value.orig_exc, ZeroDivisionError)
pytest.ini
[pytest]
pythonpath = .

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