Speed up construction of single qubit pauli operations - cirq.X(q) (#…

…6316)

Co-authored-by: Matthew Neeley <maffoo@google.com>

cirq-core/cirq/ops/pauli_gates.py

@@ -15,6 +15,7 @@
 from typing import Any, cast, Tuple, TYPE_CHECKING, Union, Dict
 
 from cirq._doc import document
+from cirq._import import LazyLoader
 from cirq.ops import common_gates, raw_types, identity
 from cirq.type_workarounds import NotImplementedType
 
@@ -29,6 +30,9 @@
     )  # pragma: no cover
 
 
+pauli_string = LazyLoader("pauli_string", globals(), "cirq.ops.pauli_string")
+
+
 class Pauli(raw_types.Gate, metaclass=abc.ABCMeta):
     """Represents the Pauli gates.
 
@@ -97,9 +101,8 @@ def on(self, *qubits: 'cirq.Qid') -> 'SingleQubitPauliStringGateOperation':
         """
         if len(qubits) != 1:
             raise ValueError(f'Expected a single qubit, got <{qubits!r}>.')
-        from cirq.ops.pauli_string import SingleQubitPauliStringGateOperation
 
-        return SingleQubitPauliStringGateOperation(self, qubits[0])
+        return pauli_string.SingleQubitPauliStringGateOperation(self, qubits[0])
 
     @property
     def _canonical_exponent(self):

cirq-core/cirq/ops/pauli_string.py

@@ -42,7 +42,7 @@
 import sympy
 
 import cirq
-from cirq import value, protocols, linalg, qis
+from cirq import value, protocols, linalg, qis, _compat
 from cirq._doc import document
 from cirq._import import LazyLoader
 from cirq.ops import (
@@ -184,7 +184,7 @@ def __init__(
         Raises:
             TypeError: If the `qubit_pauli_map` has values that are not Paulis.
         """
-        if qubit_pauli_map is not None:
+        if _compat.__cirq_debug__.get() and qubit_pauli_map is not None:
             for v in qubit_pauli_map.values():
                 if not isinstance(v, pauli_gates.Pauli):
                     raise TypeError(f'{v} is not a Pauli')