Raise DefinitionSyntaxError instead of AssertionError for an operator-only expression (#2124)

[vineethsaivs]

Fixes #2124.

Parsing an expression that is only an operator raised a bare AssertionError rather than a pint error:

import pint
ureg = pint.UnitRegistry()
ureg('-')   # before: AssertionError()
ureg('*')   # before: AssertionError()

In pint_eval._build_eval_tree, an operator with no operand leaves result is None, which tripped assert result is not None. The surrounding malformed-token paths already raise DefinitionSyntaxError, so this does the same:

ureg('-')   # after: pint.errors.DefinitionSyntaxError: missing operand

Valid expressions are unchanged (ureg('3 m / s'), ureg('m**2'), ...), and an unknown unit name still raises UndefinedUnitError (ureg('foobar')).

Added test_build_eval_tree_missing_operand_raises_definition_syntax_error (parametrized over both tokenizers and several operators); it fails on master (AssertionError) and passes with this change. All 145 tests in test_pint_eval.py pass.

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Disclosure: prepared with AI assistance; reviewed by me and I can explain every line.

[codspeed-hq]

Merging this PR will degrade performance by 24.81%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 52 regressed benchmarks
✅ 396 untouched benchmarks
⏩ 448 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	test_parse_expression[True-meter]	59.6 µs	84.6 µs	-29.53%
❌	WallTime	test_parse_expression[True-kilometer]	61.6 µs	86.4 µs	-28.74%
❌	WallTime	test_parse_expression[True-angstrom]	61.7 µs	86.6 µs	-28.72%
❌	WallTime	test_parse_expression[False-second]	60.5 µs	84.7 µs	-28.61%
❌	WallTime	test_parse_expression[False-minute]	59.9 µs	83.8 µs	-28.56%
❌	WallTime	test_parse_expression[True-minute]	60.1 µs	83.9 µs	-28.41%
❌	WallTime	test_parse_expression[False-millisecond]	62.8 µs	87.6 µs	-28.32%
❌	WallTime	test_parse_expression[False-meter]	59.6 µs	83.2 µs	-28.31%
❌	WallTime	test_parse_expression[False-angstrom]	61.5 µs	85.4 µs	-28.01%
❌	WallTime	test_parse_expression[True-millisecond]	62.3 µs	86.2 µs	-27.75%
❌	WallTime	test_parse_expression[True-second]	60.1 µs	83.1 µs	-27.73%
❌	WallTime	test_getitem[True-angstrom]	36.8 µs	50.5 µs	-27.18%
❌	WallTime	test_parse_expression[False-kilometer]	62.3 µs	85.4 µs	-27.02%
❌	WallTime	test_getitem[True-second]	35.8 µs	49 µs	-26.84%
❌	WallTime	test_getitem[False-angstrom]	37.2 µs	50.9 µs	-26.81%
❌	WallTime	test_op2[equal-keys5]	42 µs	57.3 µs	-26.78%
❌	WallTime	test_getitem[False-second]	35.6 µs	48.5 µs	-26.59%
❌	WallTime	test_finding_meter_getitem	34.9 µs	47.5 µs	-26.5%
❌	WallTime	test_getitem[True-meter]	34.7 µs	47.2 µs	-26.4%
❌	WallTime	test_getitem[True-minute]	35.6 µs	48.1 µs	-25.96%
...	...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing vineethsaivs:fix/eval-tree-missing-operand (62fdd54) with master (7a927b4)}

Footnotes

1. 448 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩