B4J Library [B4J, B4XPages] B4J as an interactive backend for Python3

Many of you already know how to do this, but I recently needed it and ran into some problems.
In the end, I had a Class that others may find useful.

My experience with this project:

1. The current version of Python is 3.12.3. Python3 has quite a few differences from Python2, and is not backwards compatible.
The PyUDP class attached is for Python3. When installing Python3, a launcher Py.exe can be accessed by a shortcut "py".

2. The python-invocation examples from the Forum are now quite old, and not based on B4XPages.

3. The interface used by this class, based on a UDP Socket, is finicky and unless everything is lined up, it does not work.

4. If you put the Python script in a file with .py extension and invoke Python with jShell, you can read the StdOut string.
But each time a dynamically created block of Python code is run, the Python interpreter has to be launched.
This does not lend itself to effective interaction between the B4J code and the Python code.

@Erel posted an example of using a UDP Socket in Python.

[RPi] - PiFace Control And Display (CAD)

jPi4J library supports the PiFace Digital extension. However it doesn't support the CAD module as the underlying library (Pi4J) doesn't support it. So we need to be a bit more creative here. We will run and interact with a simple Python script using UDP. Note that an external network is not...

www.b4x.com

It was for controlling the RPi CAD. I used the same principle.
The idea is to launch Python only once, use it interactively, and finally kill it when not needed anymore.

The flow is:
1. Define and initialize an instance of PyUDP
2. Write some Python code. There are several options (Notepad++, save file, read file), or write it in a smart string in the B4J IDE
3. Run the code as string, wait for results, respond appropriately - possibly by running more Python code
4. When Python is no more needed run .Kill

Here are 2 examples:

Private Sub B4XPage_Created (Root1 As B4XView)
    Root = Root1
    B4XPages.AddPageAndCreate("SP", SP)        'Simple XY plot used in second example
    B4XPages.AddPageAndCreate("Py", Py)
    Wait for Py_Ready
    Sleep(0)                                'Allow B4XMainPage to finish loading
    B4XPages.ShowPage("Py")

'Do Fibonacci sequence
    Py.Run($"
# Fibonacci numbers module
import sys
def fib(n):
    result = []
    a, b = 0, 1
    while a < n:
        result.append(a)
        a, b = b, a+b
    return result

print(fib(100))
print(fib(200))
print(fib(300))
print("")
"$)
    wait for Py_Complete

'Without killing Py do Linear Regression

    Dim x As String = "5,7,8,7,2,17,2,9,4,11,12,9,6"
    Dim y As String = "99,86,87,88,111,86,103,87,94,78,77,85,86"

    Py.Run($"
# Linear Regression Module
import numpy as np
from scipy import stats

x = np.array([${x}])
y = np.array([${y}])
n = x.size

slope, intercept, r, p, std_err = stats.linregress(x, y)
print("n, intercept, slope, r, p")
print( n, intercept, slope, r, p, sep=", ")
    "$)
    wait for Py_Complete
   
    plotResults(x, y, Py.GetResults)

    Py.Kill
End Sub

These examples using PyUDP illustrate the typical functions of a backend - the Python results are shown in a UI.

Note 1: I expect that most of the logic and all of the standard input and standard output of Python code would be handled by B4J code.
Standard input of a value like ans=input() would become ans=value; standard output stays the same as print()

Note 2: The server uses 'exec' to execute served code. The input() function of Python is a security risk and is therefore disabled.
It is not needed in this approach (see above note 1). Since all arguments in the server exec come directly from your B4J code,
security is not at risk. The UDPServer.py file itself is deleted after the server is invoked - althought this is unnecessary.

Note 3: It is important to kill the PyUDP server, since an already running Python prevents a new PyUDP from being created (things don't work)
In the example code, the PyUDP is killed on a close request by B4MainPage.

 

[Daestrum]

Looks easier than using scriptengine and JPython, plus you get the real libraries to use.

Well done.

 

[William Lancee]

@Daestrum : you get the real libraries to use

Yes, it is the same as running Python from a command window.

 

[epiCode]

When I run the sample program in B4J - it opens up main window and then another windows called py with just first line of python code to be executed and then does nothing. What could I be missing ?

WARNING: package com.sun.javafx.embed.swing.oldimpl not in javafx.swing
Waiting for debugger to connect...
Program started.
starting UDPServer
Call B4XPages.GetManager.LogEvents = True to enable logging B4XPages events.
Process completed

 

[William Lancee]

I'm checking for possibilities.
Do you have Python3 installed? If not then the shortcut in the shell statement "py" does not work.

I just ran the example and runs fine in my setup.

Edit: this is not the source of your problem - you wouldn't get the results you describe.

 

[William Lancee]

Another possible problem could be indentation. In Python indentation has critical meaning and has be consistently
TABs or Spaces but not both. In my example I have tabs. Does the IDE change that to spaces in your setup?

Edit: This is still a possibility. The server Python code uses TABs and therefore all code needs to use TABs as indents

 

[William Lancee]

You could try the full path in B4XPage_Created of PyUDP:

'where .... is  your user name

Dim ppath As String = "C:\Users\....\AppData\Local\Programs\Python\Python312\python.exe"
shl.Initialize("shl", ppath, Array(File.Combine(File.DirApp, "UDPServer.py")))

'instead of
'shl.Initialize("shl", "py", Array(File.Combine(File.DirApp, "UDPServer.py")))

Edit: See #5

 

[epiCode]

Do you have Python3 installed? If not then the shortcut in the shell statement "py" does not work.

python 3.10
Python is set in environment and path

c:\>where python
c:\python\python310\python.exe

Adding absolute python path as suggested by you in #7 made it work.

Thanks a lot @William Lancee