self'/classess on RPi

Question

Here is couple of my code: (main class)

    import RPi.GPIO as GPIO
    import os
    import pigpio
    import subprocess
    from time import sleep
    GPIO.setmode(GPIO.BCM)
    GPIO.setwarnings(False)
    pigpio.exceptions = False
    pi = pigpio.pi()


    class FORWARD(object):
            def __init__(self):
                    self.GPIO = GPIO
                    self.GPIO_TRIGGER = GPIO_TRIGGER
                    self.GPIO_ECHO = GPIO_ECHO


            def setup():
                    ENGA = 4
                    ENGB = 17
                    ENABLEA = 23
                    ENABLEB = 24
                    GPIO.setup(ENGA, GPIO.OUT)
                    GPIO.setup(ENGB, GPIO.OUT)
                    GPIO.setup(ENABLEA, GPIO.OUT)
                    GPIO.setup(ENABLEB, GPIO.OUT)
                    GPIO_TRIGGER = 14
                    GPIO_ECHO = 11


            def frwd(self):
                    GPIO.setup(GPIO_TRIGGER, GPIO.OUT)
                    GPIO.setup(GPIO_ECHO, GPIO.OUT)
                    GPIO.output(ENABLEB, True)
                    GPIO.output(ENABLEA, True)
                    GPIO.output(ENGA, False)
                    GPIO.output(ENGB, False)

and the sec. file:

    from ctrl import FORWARD
    run = FORWARD()
    run.setup()
    run.frwd()

And here is my question: Why it's not working? I am receiving error with the global names:

    Traceback (most recent call last):
    File "go.py", line 2, in <module>
    run = FORWARD()
    File "/root/ctrl.py", line 22, in __init__
    self.GPIO_TRIGGER = GPIO_TRIGGER
    NameError: global name 'GPIO_TRIGGER' is not defined

My point is - i just want to make a 'setup' function in which i will define PINS and GPIO outputs, then i want to define more methods (e.g - backward, stop, etc.). So what's about these selfs or how should it write?

Regards, Maciej

Answer 1

You should read the Python tutorial on classes. https://docs.python.org/2/tutorial/classes.html

The first argument to every method on a class is an instance of that class. It's convention to refer to this variable as self. So your setup method should really have self as an argument.

The specific error you are receiving is with the line:

self.GPIO_TRIGGER = GPIO_TRIGGER

You're attempting to set the GPIO_TRIGGER property of the instance to the variable GPIO_TRIGGER. However, the GPIO_TRIGGER hasn't been defined within the scope that the class is defined. i.e. you haven't defined it anywhere, and haven't imported it from somewhere else. You'll have similar issues with GPIO_ECHO.

Presumably you need to define these variables yourself, e.g.

GPIO_TRIGGER = 7
GPIO_ECHO = 8

Adjust accordingly to the pins you're using.