================
Using the PyTask
================

The PyTask allows you to call any Python function. That function can (optionally) return a *FWAction* to perform dynamic actions (e.g., see the :doc:`Guide to Writing Firetasks </guide_to_writing_firetasks>`). Thus, with the PyTask you can basically do anything!

Required parameters
===================

* **func** *(str)*: a fully qualified python method. If the function is part
  of a module then the name specification is something like ``module.function``,
  e.g. ``json.dump``, or ``shutil.copy``, or a user function.

Optional parameters
===================

* **args** *(list)*: a list of positional arguments to feed into the function. Default is an empty list.
* **kwargs** *(dict)*: a dictionary of keyword arguments. Default is empty.
* **auto_kwargs** *(bool)*: If ``True``, all other parameters not starting with
  "_" are automatically supplied as keyword args.
* **stored_data_varname** *(str)*: If this is a string that does not evaluate
  to ``False``, the output of the function will be stored as
  ``FWAction(stored_data={stored_data_varname: output})``.
  The name is deliberately long to avoid potential name conflicts.
* **inputs** *(list)*: a list of **spec** keys which will be used to pass
  data from **spec** to the function as positional arguments; the so generated
  arguments list will be appended to ``args``. Default is an empty list.
* **outputs** *(list)*: a list of **spec** keys that will be used to pass
  returned values from the function to the **spec** of current and of child
  Fireworks; If the list is empty (``[]``) or not specified (``None``) then the
  output of the function will not be stored/passed. Default is an empty list.
* **chunk_number** *(int)*: a serial number of the Firetask within a group
  of Firetasks generated by a ForeachTask. If ``chunk_number`` is not ``None``
  the output will be merged with the output of the other Firetasks in the group
  into a list under the key specified in ``outputs``. Default is ``None``.


Examples
========

Example 1: Using PyTask with static arguments
---------------------------------------------

Here is an example of defining a PyTask that sleeps for 5 seconds::

    fw_timer = Firework(PyTask(func='time.sleep',args=[5]))

Note that you can call any Python function this way!


Example 2: Redirecting data from and to **spec**
------------------------------------------------

Here is an example of using PyTask in a dataflow context::

    fws:
    - fw_id: 1
      name: Grind coffee
      spec:
        _tasks:
        - _fw_name: PyTask
          func: auxiliary.printurn
          inputs: [coffee beans]
          outputs: [coffee powder]
        coffee beans: best selection
    - fw_id: 2
      name: Brew coffee
      spec:
        _tasks:
        - _fw_name: PyTask
          func: auxiliary.printurn
          inputs: [coffee powder, water]
          outputs: [pure coffee]
        water: workflowing water
    links:
      '1': [2]
    metadata: {}
    name: Simple coffee workflow

In this example the function ``auxiliary.printurn`` prints and returns all
its arguments::

    def printurn(*args):
        result = []
        for arg in args:
            if isinstance(arg, list) and len(arg) == 1:
                result.append(arg[0])
            else:
                result.append(arg)
        if len(result) == 1:
            result = result[0]
        print(result)
        return result

The module ``auxiliary``, i.e. the file ``auxiliary.py`` must be in
``$PYTHONPATH``.
