.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "starterkit/Ex02_basics_b2luigi_require.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_starterkit_Ex02_basics_b2luigi_require.py>`
        to download the full example code.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_starterkit_Ex02_basics_b2luigi_require.py:


.. _exercise02_label:

Building a Dependency Graph with ``b2luigi``
============================================

.. hint::
    This example demonstrates the third key method of a ``b2luigi`` task: the ``requires`` method.
    This method is used to define dependencies between different tasks.

The ``requires`` method is used to define dependencies between different tasks.
The method should return iterable instances of the required tasks.
This task will be scheduled to run after the output of the required task is completed.
The output is considered completed if the output files are present.

.. warning::
    There is no check of the content of the output files!
    Consequently, the previous task can be forced to rerun by deleting the output.

To show the functionality of the ``requires`` method, we will create a second task that depends on the first task.
For this, we define the first task in the same way as in the previous example.

.. GENERATED FROM PYTHON SOURCE LINES 23-37

.. code-block:: Python

    import b2luigi


    class MyTask(b2luigi.Task):
        parameter = b2luigi.IntParameter()

        def run(self):
            with open(self.get_output_file_name("output.txt"), "w") as f:
                f.write(f"{self.parameter}")

        def output(self):
            yield self.add_to_output("output.txt")



.. GENERATED FROM PYTHON SOURCE LINES 38-52

We again make use of the :meth:`b2luigi.Task.add_to_output` method to add
another output file in the now existing directory structure. In this
example, the output is structured as follows:

.. code-block:: none

  parameter=1/
    output.txt
    output2.txt

We use :meth:`b2luigi.Task.get_input_file_names` to get the file name of
the output file of the required task. The method returns an iterable
of file names. In this case, we know that there is only one file
present and we can access it by accessing the first element.

.. GENERATED FROM PYTHON SOURCE LINES 55-72

.. code-block:: Python

    class MyOtherTask(b2luigi.Task):
        parameter = b2luigi.IntParameter()

        def requires(self):
            return MyTask(parameter=self.parameter)

        def output(self):
            yield self.add_to_output("output2.txt")

        def run(self):
            with open(self.get_input_file_names("output.txt")[0], "r") as f:
                number = int(f.read())

            with open(self.get_output_file_name("output2.txt"), "w") as f:
                f.write(f"{number**2}")



.. GENERATED FROM PYTHON SOURCE LINES 73-77

For the process method, we now have to call the new task. The scheduler
will automatically resolve the dependencies between the tasks. If the
output of ``MyTask`` is not present, it will be executed first. As soon as,
the output is present, ``MyOtherTask`` will be executed.

.. GENERATED FROM PYTHON SOURCE LINES 77-80

.. code-block:: Python

    if __name__ == "__main__":
        b2luigi.process(MyOtherTask(parameter=1))


.. GENERATED FROM PYTHON SOURCE LINES 81-84

You can try to adjust the value in `parameter=1/output.txt`, delete the
output of ``MyOtherTask`` and rerun the script to see how this affects the
result of the tasks.


.. _sphx_glr_download_starterkit_Ex02_basics_b2luigi_require.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: Ex02_basics_b2luigi_require.py <Ex02_basics_b2luigi_require.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: Ex02_basics_b2luigi_require.zip <Ex02_basics_b2luigi_require.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

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