Add caching to value_equality_values decorator for auto generated m…

…ethods. (#6275)

* Add caching to value_equality_values decorator for auto generated methods.

* Fix pylint and formatting errors

* Address nits, fix bugs and make PauliSum unhashable

cirq-core/cirq/ops/dense_pauli_string.py

@@ -570,6 +570,9 @@ def copy(
     def __str__(self) -> str:
         return super().__str__() + ' (mutable)'
 
+    def _value_equality_values_(self):
+        return self.coefficient, tuple(PAULI_CHARS[p] for p in self.pauli_mask)
+
     @classmethod
     def inline_gaussian_elimination(cls, rows: 'List[MutableDensePauliString]') -> None:
         if not rows:

cirq-core/cirq/ops/linear_combinations.py

@@ -357,7 +357,7 @@ def _pauli_string_from_unit(unit: UnitPauliStringT, coefficient: Union[int, floa
     return PauliString(qubit_pauli_map=dict(unit), coefficient=coefficient)
 
 
-@value.value_equality(approximate=True)
+@value.value_equality(approximate=True, unhashable=True)
 class PauliSum:
     """Represents operator defined by linear combination of PauliStrings.
 

cirq-core/cirq/value/value_equality_attr.py

@@ -17,7 +17,7 @@
 
 from typing_extensions import Protocol
 
-from cirq import protocols
+from cirq import protocols, _compat
 
 
 class _SupportsValueEquality(Protocol):
@@ -221,13 +221,21 @@ class return the existing class' type.
             )
     else:
         setattr(cls, '_value_equality_values_cls_', lambda self: cls)
-    setattr(cls, '__hash__', None if unhashable else _value_equality_hash)
+    cached_values_getter = values_getter if unhashable else _compat.cached_method(values_getter)
+    setattr(cls, '_value_equality_values_', cached_values_getter)
+    setattr(cls, '__hash__', None if unhashable else _compat.cached_method(_value_equality_hash))
     setattr(cls, '__eq__', _value_equality_eq)
     setattr(cls, '__ne__', _value_equality_ne)
 
     if approximate:
         if not hasattr(cls, '_value_equality_approximate_values_'):
-            setattr(cls, '_value_equality_approximate_values_', values_getter)
+            setattr(cls, '_value_equality_approximate_values_', cached_values_getter)
+        else:
+            approx_values_getter = getattr(cls, '_value_equality_approximate_values_')
+            cached_approx_values_getter = (
+                approx_values_getter if unhashable else _compat.cached_method(approx_values_getter)
+            )
+            setattr(cls, '_value_equality_approximate_values_', cached_approx_values_getter)
         setattr(cls, '_approx_eq_', _value_equality_approx_eq)
 
     return cls