Picamera2 commands between modules

[GigiG]

Hi all, please be patience, I'm a newbie
I started to write a simple program to allow comunication and control between App Inventor and Python on RPi4 with Camera, via Bluetooth.
Comunication seems to work just fine. Now, I'm facing with variables implementation issues. Basically, I have a glvars.py file:

from picamera2 import Picamera2, Preview

# Camera init
cam = Picamera2()
camera_config = cam.create_preview_configuration()
cam.configure(camera_config)
cam.start_preview(Preview.DRM, x=0, y=0, width=1024, height=600)
cam.start()
cam.stop_preview()

for 'global' variables (I'm not sure this is a good or bad idea) and a myModules.py file with:

import glvars

def camExec(*args): # Camera commands
    clientSock = args[0]
    cmd = args[1]
    # line with Picamera2 commands follows

my App Inventor app send strings to RPi via bluetooth and the running python scripts should interpret these strings as Picamera2 commands. For example, if I send the string 'start_preview(True)' (cmd = args[1] variable above), the resulting command for Picamera2 should be:

    glvars.cam.start_preview(True)

that works just fine, but

glvars.cam.cmd

doesn't.
EDIT:

def camExec(*args): # Camera commands
    clientSock = args[0]
    cmd = args[1]
    # glvars.cam.start_preview(True) # works
    return getattr(glvars.cam, cmd)

give me the error:

AttributeError: 'Picamera2' object has no attribute 'start_preview(True)'

I tried several string formatting methods but no success. How should I format the final command line in Python?
Thanks

[deanhystad]

It looks like you have something you made using app inventor sending a string to a python program. The python program executes the command represented by that string.

You could use eval() to execute the command.

eval("glvars.cam.start_preview(True)")

That is easy, but potentially dangerous. eval will execute any string it is passed, including commands to delete all files on your computer or install malware from a website. Unless you know for sure that there is no way malicious commands can be sent to your program, you should pass on eval.

getattr is on the right track. getattr(glvars.cam, "start_preview") returns the Picamera2.start_preview method. You call the method like this:

method = getattr(glvars, cam, "start_preview")  # get the method
method() # call the method

That doesn't call the method with arguments. You need to write additional code to accept the arguments and convert them from strings to whatever objects they are supposed to be. Maybe you could create a link between the commands you want to execute and the processing required to convert arguments from strings to ints, floats, bools. etc.

You could put the commands and argument processing code in a dictionary. This dictionary "exports" two commands: glcvars.cmd.start_preview which takes one bool argument, and glvars.cmd.stop which takes no arguments.

commands = {
    "start_preview": {"receiver": glvars.cmd.start_preview, "converters": [bool]},
    "stop": {"receiver": glvars.cmd.stop, "converters": []},
}

To make the command strings easier to parse you could pass a csv string instead of something that looks like python code.

cmd_str = "start_preview,True"  # command str app inventor sends.
cmd_name, *arg_strings = cmd_str.split(",")  # Split into the command and arguments.
command = commands[cmd_name]  # Lookup information for the command.
args = [converter(arg) for converter, arg in zip(command["converters"], arg_strings)  # Call converters to change "True" to True.
command["receiver"](*args)  #Call the method with arguments.