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The ImportError cannot be more annoying during development,

root@fx30s:~# python3
Python 3.5.2 (default, Oct  2 2020, 02:54:47)
[GCC 6.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import PythonModuleINeedUrgently
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named 'PythonModuleINeedUrgently'
>>>

Thanks to the beloved Python community, we have many modules around and most of them can be easily installed via pip. Unfortunately, when it comes to Yocto world, the focus is to keep the core image size small, and mostly you may have to rebuild the image to include a Python module. And often, the partition set up just does not have enough space to accommodate the new image which is far from ideal.

However, if you have installed PyRTE Legato app, you might be able to install the module you need without rebuilding the Linux. Here is an example to install PyModbus

Download the source code

Most Python modules are open source and you can get the source code from their repository. For PyModbus, we can download the compressed source code here, https://github.com/riptideio/pymodbus/releases/tag/v2.5.2

Decompressed the code, you would be able to see the module is fully written in Python, and the code directory is like this, pymodbus-2.5.2\pymodbus-2.5.2\pymodbus

Copy the code into device

ssh in FX30, create myapp directory if not yet, and in myapp, create another directory called site-packages,

root@fx30s:~/myapp/site-packages# pwd
/home/root/myapp/site-packages

Use SCP or WinSCP to copy the pymodbus folder into site-packages, then you should have the below on FX30,

root@fx30s:~/myapp/site-packages/pymodbus# ls -l
-rw-r--r--    1 root     root           899 Jul 30 03:18 __init__.py
-rw-r--r--    1 root     root          8596 Jul 30 03:18 bit_read_message.py
-rw-r--r--    1 root     root          9205 Jul 30 03:18 bit_write_message.py
drwxr-xr-x    3 root     root           512 Jul 30 07:34 client
-rw-r--r--    1 root     root          3440 Jul 30 03:18 compat.py
-rw-r--r--    1 root     root          7485 Jul 30 03:18 constants.py
drwxr-xr-x    3 root     root           520 Jul 30 07:34 datastore
-rw-r--r--    1 root     root         23243 Jul 30 03:18 device.py
-rw-r--r--    1 root     root         29260 Jul 30 03:18 diag_message.py
-rw-r--r--    1 root     root          6382 Jul 30 03:18 events.py
-rw-r--r--    1 root     root          3691 Jul 30 03:18 exceptions.py
-rw-r--r--    1 root     root         12413 Jul 30 03:18 factory.py
-rw-r--r--    1 root     root         14237 Jul 30 03:18 file_message.py
drwxr-xr-x    2 root     root           608 Jul 30 07:34 framer
-rw-r--r--    1 root     root          8363 Jul 30 03:18 interfaces.py
drwxr-xr-x    2 root     root           304 Jul 30 07:34 internal
-rw-r--r--    1 root     root          7939 Jul 30 03:18 mei_message.py
-rw-r--r--    1 root     root         14969 Jul 30 03:18 other_message.py
-rw-r--r--    1 root     root         17008 Jul 30 03:18 payload.py
-rw-r--r--    1 root     root          7966 Jul 30 03:18 pdu.py
-rw-r--r--    1 root     root         12958 Jul 30 03:18 register_read_message.py
-rw-r--r--    1 root     root         12058 Jul 30 03:18 register_write_message.py
drwxr-xr-x    4 root     root           432 Jul 30 07:34 repl
drwxr-xr-x    3 root     root           512 Jul 30 07:34 server
-rw-r--r--    1 root     root         22806 Jul 30 03:18 transaction.py
-rw-r--r--    1 root     root          7578 Jul 30 03:18 utilities.py
-rw-r--r--    1 root     root          1550 Jul 30 03:18 version.py
root@fx30s:~/myapp/site-packages/pymodbus#

Reboot the device, you should have your PyModbus ready to use.

root@fx30s:~# python3
Python 3.5.2 (default, Oct  2 2020, 02:54:47)
[GCC 6.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pymodbus.client.sync import ModbusTcpClient
>>> client = ModbusTcpClient('127.0.0.1')
>>> exit()
root@fx30s:~#

Great, now you have installed the PyModbus module(hopefully in minutes). Just note, the module itself is fully written in Python which makes the installation more easier. Otherwise, there might be cross compiling and linking involved. We can explore this part further if there are demands.