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PyModbus - trigger an action/function when holding register is read

I've setup a running Pymodbus server based on the 'Updating Server' (v2.5.3) example.
https://pymodbus.readthedocs.io/en/v2.5.3/source/example/updating_server.html
Everything works ok.
Now i want to trigger a function (which will simply increment a value by 1), when (any) client is requesting to read/poll the contents of the holdingregisters.
Console output, when client requests function code 3
My knowledge of Pymodbus is limited, so any help would be great.

There is likely a way to overload the StartTCPServer function, though looking through the source code it seems tricky since it's built on asyncio (not like that's bad, just less conducive to easily overloading this one thing you're trying to do).
However there are several modbus libraries you can use, and sourceperl's pyModbusTCP makes it very easy to do exactly what you're trying to do. Check out the example https://github.com/sourceperl/pyModbusTCP/blob/master/examples/server_change_log.py
There they use the built-in hooks for doing just that, but you can get easily overload any part of the server if you wanted to for example capture the incoming modbus message before it gets processed.

How we can automate python client application which is used an an interface for user to call APIs

We have made an python client which is used as an interface for user. some function is defined in the client which internally calls the APIs and give output to users.
My requirement is to automate the python client - functions and validate the output.
Please suggest tools to use.

There are several ways to do that:
You can write multiple tests for your application as the test cases which are responsible to call your functions and get the result and validate them. It calls the "feature test". To do that, you can use the python "unittest" library and call the tests periodically.
If you have a web application you can use "selenium" to make automatic test flows. (Also you can run it in a docker container virtually)
The other solution is to write another python application to call your functions or send requests everywhere you want to get the specific data and validate them. (It's the same with the two other solutions with a different implementation)

The most straightforward way is using Python for this, the simplest solution would be a library like pytest. More comprehensive option would be something like Robot framework
Given you have jmeter in your tags I assume that at some point you will want to make a performance test, however it might be easier to use Locust for this as it's pure Python load testing framework.
If you still want to use JMeter it's possible to call Python programs using OS Process Sampler

Listening to the Output of a Web Application

I have been trying, in vain, to make a program that reads text out loud using the web application found here (http://www.ispeech.org/text.to.speech.demo.php). It is a demo text-to-speech program, that works very well, and is relatively fast. What I am trying to do is make a Python program that would input text to the application, then output the result. The result, in this case, would be sound. Is there any way in Python to do this, like, say, a library? And if not, is it possible to do this through any other means? I have looked into the iSpeech API (found here), but the only problem with it is that there is a limited number of free uses (I believe that it is 200). While this program is only meant to be used a couple of times, I would rather it be able to use the service more then 200 times. Also, if this solution is impractical, could anyone direct me towards another alternative?
# AKX I am currently using eSpeak, and it works well. It just, well, doesn't sound too good, and it is hard to tell at times what is being said.

If using iSpeech is not required, there's a decent (it's surely not as beautifully articulated as many commercial solutions) open-source text-to-speech solution available called eSpeak.
It's usable from the command line (subprocess with Python), or as a shared library. It seems there's also a Python wrapper (python-espeak) for it.
Hope this helps.

OK. I found a way to do it, seems to work fine. Thanks to everyone who helped! Here is the code I'm using:
from urllib import quote_plus
def speak(text):
import pydshow
words = text.split()
temp = []
stuff = []
while words:
temp.append(words.pop(0))
if len(temp) == 24:
stuff.append(' '.join(temp))
temp = []
stuff.append(' '.join(temp))
for i in stuff:
pydshow.PlayFileWait('http://api.ispeech.org/api/rest?apikey=8d1e2e5d3909929860aede288d6b974e&format=mp3&action=convert&voice=ukenglishmale&text='+quote_plus(i))
if __name__ == '__main__':
speak('Hello. This is a text-to speech test.')
I find this ideal because it DOES use the API, but it uses the API key that is used for the demo program. Therefore, it never runs out. The key is 8d1e2e5d3909929860aede288d6b974e.
You can actually test this at work without the program, by typing the following into your address bar:
http://api.ispeech.org/api/rest?apikey=8d1e2e5d3909929860aede288d6b974e&format=mp3&action=convert&voice=ukenglishmale&text=
Followed by the text you want to speak. You can also adjust the language, by changing, in this case, the ukenglishmale to something else that iSpeech offers. For example, ukenglishfemale. This will speak the same text, but in a feminine voice.
NOTE: Pydshow is my wrapper around DirectShow. You can use yours instead.

The flow of your application would be like this:
Client-side: User inputs text into form, and form submits a request to server
Server: may be python or whatever language/framework you want. Receives http request with text.
Server: Runs text-to-speech either with pure python library or by running a subprocess to a utility that can generate speech as a wav/mp3/aiff/etc
Server: Sends HTTP response back by streaming file with a mime type to Client
Client: Receives the http response and plays the content
Specifically about step 3...
I don't have any particular advise on the most articulate open source speech synthesizing software available, but I can say that it does not have to necessarily be pure python, or even python at all for that matter. Most of these packages have some form of a command line utility to take stdin or a file and produce an audio file as output. You would simply launch this utility as a subprocess to generate the file, and then stream the file back in your http response.
If you decide to make use of an existing web service that provides text-to-speech via an API (iSpeech), then step 3 would be replaced with making your own server-side http request out to iSpeech, receiving the response and pretty much forwarding that response back to the original client request, like a proxy. I would say the benefit is not having to maintain your own speech synthesis solution or getting better quality that you could from an open source... but the downside is that you probably will have a bit more latency in your response time since your server has to make its own external http request and download the data first.

How to expose data to zabbix

Here is my goal: I would like to be able to report various metrics to zabbix so that we can display the graphs on a web page.
These metrics include:
latency per soap service submission
various query results from one or more databases.
What things do I need to write and/or expose? Or is the zabbix server going to go and get it from an exposed service somewhere?
I've been advised that a script that returns a single value will work, but I'm wondering if that's the right way.

I can offer 2 suggestions to get the metrics into Zabbix:
Use the zabbix_sender binary to feed the data from your script directly to the Zabbix server. This allows your script to call on it's own interval and set all the parameters needed. You really only need to know the location to the zabbix_sender binary. Inside the Zabbix server interface, you would create items with the type of Zabbix trapper. This is the item type which receives values send from the zabbix_sender. You make up the key name and it has to match.
The second way you could do this is to specify a key name and script/binary inside the zabbix_agentd.conf file. Every time the Zabbix server requests this item the script would be called and the data from the script recorded. This allows you to set the intervals in the Zabbix item configuration rather than forcing you to run your script on its own intervals. However, you would need to add this extra bit of information to your zabbix_agentd.conf file for every host.
There may be other ways to do this directly from Python (zabbix_sender bindings for Python maybe?). But these are the 2 ways I have used before which work well. This isn't really Python specific. But you should be able to use zabbix_sender in your Python scripting. Hope this information helps!
Update: I also remembered that Zabbix was working on/has a API (JSON/RPC style). But the documentation site is down at the moment and I am not sure if the API is for submitting item data or not. Here is the Wiki on the API: http://www.zabbix.com/wiki/doc/api
And a project for Python API: https://github.com/gescheit/scripts/tree/master/zabbix/
There seems to be little documentation on the API as it is new as of Zabbix version 1.8

Actually there is a python binding for zabbix_sender. http://pypi.python.org/pypi/zbxsend

Which of two methods of using python within erlang should I use?

I have a project where I use erlang to aggregate RSS, and I use python to process the RSS feeds.
Method 1:
Use an erlang port, using erlport.org, to call python.
I'm not sure how to design the python code to be asyncrhonous using erlport.
Method 2:
Use erlang to call on a RESTful interface with Tornado that does the processing (asynchro downloading of urls -- asynchro procssing)

I haven't worked Erlang<->Python before but erlport.org seems promising. I would try that first before getting into greasiness with REST and what not. I.e. I didn't provide and answer but a recommendation :)