Developer data plugin tutorial - Float summation

Now that you have writen your first AiiDA plugin, we can try to extend it to see how we can introduce different type of parameters and how the plugins have to be modified to encompass these changes.

Introducing a new data type

We will start by describing what is a data plugin, and by creating a new one.

A data plugin is a subclass of Data. What you have to do is just to define a subclass with a suitable name inside the aiida/orm/data folder (with the same name convention of Calculation plugins: the class should be called NameData (with Name being a name of your choice) and put in a aiida/orm/data/ file. In the class, you should provide methods that the end user should use to store high-level objects (for instance, for a crystal structure, there can be a method for setting the unit cell, one for adding an atom in a given position, ...). Internally, you should choose where to store the content. There are two options:

  • In the AiiDA database. This is useful for small amounts of data, that you plan to query. In this case, use self._set_attr(attr_name, attr_value) to store the required value.
  • In the AiiDA file repository (as a file on the disk). This is suitable for big files and quantities that you do not want to query. In this case, access the folder using self.folder and use the methods of self.folder to create files, subfolders, ...

Of course, it is also good practice to provide “getter” methods to retrieve the data in the database and return it back to the user. The idea is that the user can operate directly only with the methods you provide, and should not need to know how you decided to store the data inside the AiiDA database.

As a simple example that we will use for the exercise below, imagine that we want to introduce a new type of data node that simply stores a float number. We will call it FloatData, and the class implementation can look like this:

from import Data

class FloatData(Data):

    def value(self):
        The value of the Float
        return self.get_attr('number')

    def value(self,value):
        Set the value of the Float

        :raise ValueError:
        self._set_attr('number', float(value))

This file should be placed under aiida/orm/data/ and it should extend the class Data.

Exercise: Modifying the calculation plugin

Your exercise consists in creating a new code plugin (let’s call it for instance floatsum) that will also perform the sum, but accept as input two FloatData node and return also a FloatData node containing the sum.

Below, you will find some hints on the parts you need to modify with respect to the previous tutorial using instead ParameterData both as inputs and outputs.


remember to create copies of your files with a new name instead of, and to change the class name accordingly.

Changes to the parser

The plugin should now return a FloatData instead of a ParameterData, therefore the parser code should contain something like the following:

output_data = FloatData()
output_data.value = out_dict["sum"]
linkname = 'output_data'

Changes to the input plugin

To be able to run your new FloatsumParser, you will need the corresponding input plugin (FloatsumCalculation). The first modification is then to link to the correct parser class:

self._default_parser = 'floatsum'

For consistency, we also want that the input plugin accepts two FloatData instead of a single ParameterData. Therefore, you have to update the retdict object accordingly:

    "float_data_1": {
       'valid_types': FloatData,
       'additional_parameter': None,
       'linkname': 'float_data_1',
       'docstring': ("The first addend"),
    "float_data_2": {
       'valid_types': FloatData,
       'additional_parameter': None,
       'linkname': 'float_data_2',
       'docstring': ("The second addend"),

You need then to change the main code to use the values obtained from the two nodes, rather than from a single node as before. This should be easy, so we leave this task to you. Note that we plan to use the same python code to actually perform the sum, so the JSON file to be generated should have the same format.

We also suggest that you add utility methods (to the benefit of the end user) to provide the addends to the code, something like:

def set_addend1(self, value):
    fl = FloatData()
    fl.value = value

and similarly for the second addend.

The final input plugin should be placed at aiida/orm/calculation/job/


The python code that actually performs the calculation does not need to be modified. We can reuse the same file, but we suggest to setup a new code in AiiDA, with a different name, using as default plugin the floatsum plugin.

Submission script

Finally, adapt your submission script to create the correct input nodes, and try to perform a sum of two numbers to verify that you did all correctly!


After placing your files, do not forget to restart the daemon so that it will recognize the files! The same should be done if you do any change to the plugin, otherwise the daemon may have cached the old file and will keep using it.