old_contents: """Workflow execution engines."""
  from resolwe.flow.engine import BaseEngine
  
  
  class BaseExecutionEngine(BaseEngine):
      """A workflow execution engine."""
  
      def evaluate(self, data):
          """Return the code needed to compute a given Data object."""
          raise NotImplementedError
  
      def get_expression_engine(self, name):
          """Return an expression engine by its name."""
          return self.manager.get_expression_engine(name)
  
      def get_output_schema(self, process):
          """Return any additional output schema for the process."""
          return []
  
      def discover_process(self, path):
          """Perform process discovery in given path.
  
          This method will be called during process registration and
          should return a list of dictionaries with discovered process
          schemas.
          """
          return []
  
      def prepare_runtime(self, runtime_dir, data):
          """Prepare runtime directory.
  
          This method should return a dictionary of volume maps, where
          keys are files or directories relative the the runtime directory
          and values are paths under which these should be made available
          to the executing program. All volumes will be read-only.
          """
  
new_contents: """Workflow execution engines."""
  from resolwe.flow.engine import BaseEngine
  
  
  class BaseExecutionEngine(BaseEngine):
      """A workflow execution engine."""
  
      def evaluate(self, data):
          """Return the code needed to compute a given Data object."""
          raise NotImplementedError
  
      def get_expression_engine(self, name):
          """Return an expression engine by its name."""
          return self.manager.get_expression_engine(name)
  
      def get_output_schema(self, process):
          """Return any additional output schema for the process."""
          return []
  
      def discover_process(self, path):
          """Perform process discovery in given path.
  
          This method will be called during process registration and
          should return a list of dictionaries with discovered process
          schemas.
          """
          return []
  
      def prepare_runtime(self, runtime_dir, data):
          """Prepare runtime directory.
  
          This method should return a dictionary of volume maps, where
          keys are files or directories relative the the runtime directory
          and values are paths under which these should be made available
          to the executing program. All volumes will be read-only.
          """
          return {}
  
subject: Return empty dictionary instead of None