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Modify IdentityGate to reject input when num_qubits is 0 #6884

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Modify IdentityGate to reject input when num_qubits is 0 #6884

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8 changes: 6 additions & 2 deletions cirq-core/cirq/ops/identity.py
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Original file line number Diff line number Diff line change
Expand Up @@ -48,10 +48,14 @@ def __init__(
applies to. The default is 2 for every qubit.

Raises:
ValueError: If the length of qid_shape doesn't equal num_qubits, or
neither `num_qubits` or `qid_shape` is supplied.
ValueError: If any of the following conditions are met:
- The length of `qid_shape` does not equal `num_qubits`.
- Neither `num_qubits` nor `qid_shape` is provided.
- `num_qubits` is 0.

"""
if num_qubits == 0:
raise ValueError('Cannot create an identity gate on 0 qubits.')
if qid_shape is None:
if num_qubits is None:
raise ValueError('Specify either the num_qubits or qid_shape argument.')
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2 changes: 2 additions & 0 deletions cirq-core/cirq/ops/identity_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,8 @@ def test_identity_init(num_qubits):
cirq.IdentityGate(5, qid_shape=(1, 2))
with pytest.raises(ValueError, match='Specify either'):
cirq.IdentityGate()
with pytest.raises(ValueError, match='Cannot create an identity gate on 0 qubits.'):
cirq.IdentityGate(0)


def test_identity_on_each():
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