Creating Python OOT with gr-modtool

This tutorial describes how to create a custom Python block and use it in a flowgraph with the following steps:
 * Create an out-of-tree module using gr_modtool
 * Create a new Python block using gr_modtool
 * Modify the Python code in a text editor so the block will function
 * Modify the YAML file so it can be displayed in GRC
 * Install and run the block in a flowgraph

A tutorial on using the Embedded Python Block in GRC is on the Creating Your First Block page. There are also additional tutorials on using tags, how to do message passing and adding vector inputs and outputs in Python blocks that will work for both the Embedded Python Block and OOT Python blocks.

The next tutorial, Creating c++ OOT with gr-modtool, describes how to build a custom c++ OOT module.

Installation Note
This tutorial was written using GNU Radio v3.10.1.1 on Ubuntu 21.10, installed using the Ubuntu PPA from the Installation Wiki Page. The basic GNU Radio install using:

$ sudo apt-get install gnuradio

does not come with the proper libraries needed to compile and install OOT modules. Consider installing the following packages before continuing:

$ sudo apt-get install gnuradio-dev cmake libspdlog-dev clang-format

Creating an OOT Module
Open a terminal and navigate to an appropriate directory for writing software, such as the home directory: $ cd $HOME

GNU Radio comes packaged with gr_modtool, software used to create out-of-tree (OOT) modules. An OOT module can be thought of as a collection of custom GNU Radio blocks. Create an OOT module named customModule using gr_modtool:

$ gr_modtool newmod customModule

The directory gr-customModule is created which contains all of the skeleton code for an OOT module, however it does not yet have any blocks. Move into the gr-customModule directory: $ cd gr-customModule

List all of the files and directories within the OOT module: $ ls

The directory listing will be as follows:

apps/ cmake/  CMakeLists.txt  docs/  examples/  grc/  include/  lib/  MANIFEST.md  python/

Creating an OOT Block
Now a block needs to be created within gr-customModule. The custom block will either add or subtract based on an input parameter, so the block is named addSubSelect:

$ gr_modtool add addSubSelect

The command will start a questionnaire about how to the block is to be defined: what block type, language and parameters:

GNU Radio module name identified: customModule ('sink', 'source', 'sync', 'decimator', 'interpolator', 'general', 'tagged_stream', 'hier', 'noblock')

Select the sync block, which produces an output for every input: Enter block type: sync

Enter python as the language: Language (python/cpp): python Language: Python Block/code identifier: addSubSelect

Enter the name or organization of the copyright holder: Please specify the copyright holder: YourName

Now enter the argument list: Enter valid argument list, including default arguments:

Enter the argument list as if writing the Python code directly. In this case the selector will determine whether or not the block performs addition or subtraction. A default argument of True is given: selector=True

Determine whether or not you want the Python quality assurance (QA) code: Add Python QA code? [Y/n] n

New files will be generated:

Adding file 'python/customModule/addSubSelect.py'... Adding file 'grc/customModule_addSubSelect.block.yml'... Editing grc/CMakeLists.txt...

Two new files were created, addSubSelect.py which defines the operation of the block and customModule_addSubSelect.block.yml which defines the interface of the block for GNU Radio Companion (GRC). The CMakeLists.txt file was modified so the two files will be installed when the module is compiled and installed.

Modifying Python File
Open the python file with a text editor: gedit python/customModule/addSubSelect.py &

The following code will be listed: ‎

Change the import statement:

Both the __init__ function and work function need to be modified. The __init__ function is modified to define the input type. The addSubSelect block will accept two complex inputs and produce a complex output, therefore the in_sig and out_sig parameters need to be changed. The selector parameter also needs to be saved as a member variable:

The work function is modified to either add or subtract the two inputs based on the selector parameter:

Press CTRL + S to save the file.

Modifying YAML File
Open the .yml file with a text editor: $ gedit grc/customModule_addSubSelect.block.yml &

The following YAML is displayed: ‎

The YAML file needs to be updated to match the addSubSelector.py file that was just modified. There is a single parameter, selector. Enter the parameter values according to:

‎

The two inputs need to be defined in the YAML:

‎

The single output needs to be defined:

‎

Press CTRL + S to save the file.

Install the Block
In the top level directory of gr_customModule, create a build directory: $ mkdir build

The directory should look like the following: apps/ build/  cmake/  CMakeLists.txt  docs/  examples/  grc/  include/  lib/  MANIFEST.md  python/

Move into the build directory, $ cd build

Then run CMake which will prepare the makefiles: cmake ..

Compile the module: $ make

Install the module with sudo: sudo make install

Multiple files will now be installed:

[ 0%] Built target pygen_apps_9a6dd [ 0%] Built target copy_module_for_tests [100%] Built target pygen_python_customModule_f524f Install the project... -- Install configuration: "Release" -- Installing: /usr/local/lib/cmake/gnuradio-customModule/gnuradio-customModuleConfig.cmake -- Installing: /usr/local/include/gnuradio/customModule/api.h -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.py -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.py -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.pyc -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.pyc -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.pyo -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.pyo -- Installing: /usr/local/share/gnuradio/grc/blocks/customModule_addSubSelect.block.yml

Using the Custom Block in a Flowgraph
Start GNU Radio Companion (GRC): $ gnuradio-companion &

The addSubSelect block will be available under the customModule drop down in the block library:



Drag the block into the workspace:



The block shows the properties as defined in the YAML file:
 * Two complex inputs, in0 and in1
 * One complex output, out0
 * A selector parameter

Double-click the block to open the properties. The selector parameter is described by the label Add (True) or Subtract (False) Selector with True as the default value:



Click OK.

Add the following blocks to the workspace:
 * Signal Source
 * Constant Source
 * Throttle
 * QT GUI Time Sink

Update the following parameters
 * Signal Source
 * Frequency: 100
 * Amplitude: 0.5
 * Constant Source
 * Constant: 0.5+0.5j

Connect the blocks according to the following flowgraph:



Running the Flowgraph
Run the flowgraph. The QT GUI Time Sink will display the following output, showing the average level of the sinusoid has been raised by 0.5+0.5j:



Edit the properties off the addSubSelect block and enter False which enables the subtraction mode:



Run the flowgraph. The QT GUI Time Sink will display the following output, showing the average level of the sinusoid has been reduced by 0.5+0.5j:



Making Changes
It is suggested to recompile and reinstall the module any time a change is made, followed by reloading the block library in GRC. This includes changes such as:
 * Number of parameters
 * Type of parameters
 * Number of input ports or output ports
 * Types of input ports or output ports
 * Modifying the YAML .yml file
 * Modifying the Python .py file

Removing and re-creating the build/ directory may be necessary before recompiling and reinstalling the module depending on the scope of the change:

$ rm -rf gr-customModule/build $ mkdir gr-customModule/build

The next tutorial, Creating c++ OOT with gr-modtool, describes how to build a custom c++ OOT module.