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Keep ResultDict unchanged when ResultDict.measurements raises an exception #8423

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Keep ResultDict unchanged when ResultDict.measurements raises an exception #8423

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6 changes: 4 additions & 2 deletions cirq-core/cirq/study/result.py
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Original file line number Diff line number Diff line change
Expand Up @@ -429,12 +429,14 @@ def params(self) -> cirq.ParamResolver:
def measurements(self) -> Mapping[str, np.ndarray]:
if self._measurements is None:
assert self._records is not None
self._measurements = {}
_measurements = {}
for key, data in self._records.items():
reps, instances, qubits = data.shape
if instances != 1:
raise ValueError('Cannot extract 2D measurements for repeated keys')
self._measurements[key] = data.reshape((reps, qubits))
_measurements[key] = data.reshape((reps, qubits))
# assign only after the _measurements are successfully resolved
self._measurements = _measurements
return self._measurements

@property
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4 changes: 3 additions & 1 deletion cirq-core/cirq/study/result_test.py
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Expand Up @@ -87,8 +87,10 @@ def test_construct_from_repeated_measurements() -> None:
'b': np.array([[[0, 0, 0]], [[1, 1, 1]]]),
},
)
with pytest.raises(ValueError):
with pytest.raises(ValueError, match="Cannot extract 2D measurements"):
_ = r.measurements
Comment on lines +90 to 91

@mhucka mhucka Oct 11, 2026

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Since the PR description says

After ResultDict.measurements raises ValueError, a second call would pass instead of raising ValueError again

should this test be repeated here? I.e.,

Suggested change
with pytest.raises(ValueError, match="Cannot extract 2D measurements"):
_ = r.measurements
with pytest.raises(ValueError, match="Cannot extract 2D measurements"):
_ = r.measurements
# Check that a second access raises ValueError again.
with pytest.raises(ValueError, match="Cannot extract 2D measurements"):
_ = r.measurements

# ensure r.measurements remains unresolved after ValueError
assert r._measurements is None
assert np.all(r.records['a'] == np.array([[[0, 0], [0, 1]], [[1, 0], [1, 1]]]))
assert np.all(r.records['b'] == np.array([[[0, 0, 0]], [[1, 1, 1]]]))
assert r.repetitions == 2
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