# Constant Tasks

The tasks in this module can be used to represent constant values.

In general, users will not instantiate these tasks by hand; they will be automatically created whenever the Prefect engine detects that a constant value is required. In many cases, Prefect will not add Constant tasks to the graph; optimizing them as flow-level attributes instead.

# Constant



(value, name=None, **kwargs)[source]

The Constant class represents a single value in the flow graph.


  • value (Any): a constant value
  • name (str): a name for the constant; defaults to "Constant[(type(value))]" if not provided
  • **kwargs (Any): kwargs to pass to the Task constructor
Constant tasks are most commonly used to prevent Prefect from creating a large number of auto-generated tasks in Python collections.




The run() method is called (with arguments, if appropriate) to run a task.

Note: The implemented run method cannot have *args in its signature. In addition, the following keywords are reserved: upstream_tasks, task_args and mapped.

If a task has arguments in its run() method, these can be bound either by using the functional API and calling the task instance, or by using self.bind directly.

In addition to running arbitrary functions, tasks can interact with Prefect in a few ways:

  • Return an optional result. When this function runs successfully, the task is considered successful and the result (if any) can be made available to downstream tasks.
  • Raise an error. Errors are interpreted as failure.
  • Raise a signal. Signals can include FAIL, SUCCESS, RETRY, SKIP, etc. and indicate that the task should be put in the indicated state.
    • FAIL will lead to retries if appropriate
    • SUCCESS will cause the task to be marked successful
    • RETRY will cause the task to be marked for retry, even if max_retries has been exceeded
    • SKIP will skip the task and possibly propogate the skip state through the flow, depending on whether downstream tasks have skip_on_upstream_skip=True.

This documentation was auto-generated from commit bf7a6b9
on February 17, 2021 at 13:58 UTC