Reject multiplication of cirq.PauliSum inside in cirq.PauliString (…

…#5010)
quantumlib · Feb 22, 2022 · 3177647 · 3177647
1 parent ca4c8e7
commit 3177647
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Showing 2 changed files with 14 additions and 8 deletions.
diff --git a/cirq-core/cirq/ops/pauli_string.py b/cirq-core/cirq/ops/pauli_string.py
@@ -42,6 +42,7 @@
 
 from cirq import value, protocols, linalg, qis
 from cirq._doc import document
+from cirq._import import LazyLoader
 from cirq.ops import (
     clifford_gate,
     common_gates,
@@ -58,6 +59,9 @@
 if TYPE_CHECKING:
     import cirq
 
+# Lazy imports to break circular dependencies.
+linear_combinations = LazyLoader("linear_combinations", globals(), "cirq.ops.linear_combinations")
+
 TDefault = TypeVar('TDefault')
 TKey = TypeVar('TKey', bound=raw_types.Qid)
 TKeyNew = TypeVar('TKeyNew', bound=raw_types.Qid)
@@ -289,17 +293,13 @@ def __truediv__(self, other):
         return NotImplemented
 
     def __add__(self, other):
-        from cirq.ops.linear_combinations import PauliSum
-
-        return PauliSum.from_pauli_strings(self).__add__(other)
+        return linear_combinations.PauliSum.from_pauli_strings(self).__add__(other)
 
     def __radd__(self, other):
         return self.__add__(other)
 
     def __sub__(self, other):
-        from cirq.ops.linear_combinations import PauliSum
-
-        return PauliSum.from_pauli_strings(self).__sub__(other)
+        return linear_combinations.PauliSum.from_pauli_strings(self).__sub__(other)
 
     def __rsub__(self, other):
         return -self.__sub__(other)
@@ -1237,7 +1237,6 @@ def _imul_helper(self, other: 'cirq.PAULI_STRING_LIKE', sign: int):
         Returns:
             self on success, NotImplemented given an unknown type of value.
         """
-
         if isinstance(other, (Mapping, PauliString, MutablePauliString)):
             if isinstance(other, (PauliString, MutablePauliString)):
                 self.coefficient *= other.coefficient
@@ -1252,7 +1251,11 @@ def _imul_helper(self, other: 'cirq.PAULI_STRING_LIKE', sign: int):
             other.gate, identity.IdentityGate
         ):
             pass
-        elif isinstance(other, Iterable) and not isinstance(other, str):
+        elif (
+            isinstance(other, Iterable)
+            and not isinstance(other, str)
+            and not isinstance(other, linear_combinations.PauliSum)
+        ):
             if sign == +1:
                 other = reversed(list(other))
             for item in other:
@@ -1265,6 +1268,7 @@ def _imul_helper(self, other: 'cirq.PAULI_STRING_LIKE', sign: int):
 
     def _imul_helper_checkpoint(self, other: 'cirq.PAULI_STRING_LIKE', sign: int):
         """Like `_imul_helper` but guarantees no-op on error."""
+
         if not isinstance(other, (numbers.Number, PauliString, MutablePauliString)):
             other = MutablePauliString()._imul_helper(other, sign)
             if other is NotImplemented:

diff --git a/cirq-core/cirq/ops/pauli_string_test.py b/cirq-core/cirq/ops/pauli_string_test.py
@@ -219,6 +219,8 @@ def test_constructor_flexibility():
         _ = cirq.PauliString('test')
     with pytest.raises(TypeError, match='S is not a Pauli'):
         _ = cirq.PauliString(qubit_pauli_map={a: cirq.S})
+    with pytest.raises(TypeError, match="cirq.PAULI_STRING_LIKE"):
+        _ = cirq.PauliString(cirq.Z(a) + cirq.Z(b))
 
     assert cirq.PauliString(cirq.X(a)) == cirq.PauliString(qubit_pauli_map={a: cirq.X})
     assert cirq.PauliString([cirq.X(a)]) == cirq.PauliString(qubit_pauli_map={a: cirq.X})