Apply arbitrary python function(s) to data.


The F(x) nodes have a similar role as the Calculator node. But where the Calculator node shines when the calculations are simple expressions, the F(x) nodes are better suited for more advanced calculations since the code may span multiple lines, include statements and may even be kept in a separate python file. You can place this python file anywhere, but it might be a good idea to keep it in the same folder as your workflow or in a subfolder to that folder.

Defining a function

The python function that should be called by the node needs to be decorated with fx.decorator. The function should also take exactly two positional arguments representing the input and output ports respectively. It is recommended to name the arguments arg and res. These variables are of the same type as the input on port2. Consult the API for that type to figure out relevant operations.

The argument to fx.decorator is a list of types (as shown in port tooltips) that you intend your script to support. When selecting functions from a file, each function is only available if the port type matches one of its types.


from sympathy.api import fx

def my_calculation(arg, res):
    spam = arg['spam']

    # My advanced calculation:
    more_spam = spam + 1

    res['more spam'] = more_spam

A quick way to get the skeleton for a function is to use the function wizard that is started by clicking File->Wizards->New Function.

Script without separate file

For short scripts that are not intended to be shared between different nodes it can convenient not having to create an external file. To enable this feature, the datasource port first needs to be deleted. Once there is no datasource port, the node will show a code editor when configured.

The format is the same, the only exception is that all calculations (matching input type) will be executed.

Debugging your functions

When the functions are in a separate file they can be debugged by following the normal process for debugging nodes. See Debugging nodes for instructions.

List behavior

The same function can be used with both F(x) and F(x) List nodes. For example a function specified to run for type ‘table’ can be used with an F(x) node connected to a single table or with an F(x) List node connected to a list of tables. In the latter case the function will be executed once per item in the list.


When Copy input is disabled (the default) the output table will be empty when the functions are run.

When the Copy input setting is enabled the entire input table will get copied to the output before running the functions in the file. This is useful when your functions should only add a few columns to a data table, but in this case you must make sure that the output has the same number of rows as the input.

Alternative function definition

Another syntax for writing a “function” is to define a class which inherits from fx.Fx. The fx.Fx class provides access to the input and output with self.arg and self.res respectively. These variables are of the same type as the input on port2. The field arg_types should contain the list of types that you intend your script to support.


from sympathy.api import fx

class MyCalculation(fx.Fx):
    arg_types = ['table']

    def execute(self):
        spam = self.arg['spam']

        # My advanced calculation:
        more_spam = spam + 1

        self.res['more spam'] = more_spam

This syntax is available mostly for backwards compatibility. For new functions it is recommended to use the syntax with decorated functions.


Input ports


0 - 1, datasource

Path to Python file with scripted functions.



Item with data to apply functions on

Output ports



Item with the results from the applied functions

Copy input (copy_input)

If enabled the incoming data will be copied to the output before running the nodes.

Select functions (selected_functions)

Choose one or many of the listed functions to apply to the content of the incoming item.

Python code (code)

Python code block, input is called arg, output res.

class node_fx_selector.Fx[source]

Related nodes

Example flows