Data plugin - Float Summation

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. 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 aiida.orm 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))

The definition of this new data type should reside below a data sub package in your plugin package, with directory structure like:

      data/  <-- Put the code here

And following lines should be there in the setup.json file:

   "entry_points": {
      "": {
         "yourplugin.float ="

See also

Please see the documentation about the entry points to learn more about the plugin system.

Exercise: Modifying the calculation plugin

Your exercise consists in creating a new calculation plugin (let’s call it for instance SumFloatCalculation) 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 tutoria using instead Dict both as inputs and outputs.


Remember to add an entry point for the SumFloatCalculation in the setup.json file and re-install the package and refresh entry points. It is up to you to either put the new class in the same or create a new The same also applies to the new parser class.

Changes to the parser

The plugin should now return a FloatData instead of a Dict, 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 = 'sum.floatsum'  # Name of the entry point

For consistency, we also want that the input plugin accepts two FloatData instead of a single Dict. 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 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 sum.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 (re)registering the entry points, do not forget to restart the daemon so that it will recognize the change! 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.