def __init__(self, execution_strategy: ExecutionStrategy) -> None:
        """Initializes transformer.

        Args:
            execution_strategy: The `ExecutionStrategy` to execute.

        Raises:
            ValueError: if execution_strategy is None.
        """

        if execution_strategy is None:
            raise ValueError('execution_strategy cannot be None')
        self.execution_strategy = execution_strategy
        self._mapping = execution_strategy.initial_mapping.copy()
Initializes transformer.

        Args:
            execution_strategy: The `ExecutionStrategy` to execute.

        Raises:
            ValueError: if execution_strategy is None.
        
cirq-core/cirq/contrib/acquaintance/executor.py