GPIO and Python - GPIO.output() not working

Question

I want to program a simple thermostat, but for some reason the commands GPIO.output(21, 1) or GPIO.output(21, 0) do not change anything.

In a similar vein, GPIO.setup(20, GPIO.OUT, initial=0) does indeed setup the pin for output, but it's automatically on, despite the initial=0...

Here's the code:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(20, GPIO.OUT, initial=0)
ventein = GPIO.output(20, 1)
ventaus = GPIO.output(20, 0)

calling ventein or ventaus further down the line does not do anything. The pin is just always on. I suspected that maybe it has something to do with user permissions, so I added both root and the user to the gpio group - didn't do anything.

Toggling the pin with the gpio command line tool works just fine. What could be the problem?

And here's the complete code, just in case:

from w1thermsensor import W1ThermSensor
import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
GPIO.setup(20, GPIO.OUT, initial=0)
ventein = GPIO.output(20, 1)
ventaus = GPIO.output(20, 0)
muro = W1ThermSensor(W1ThermSensor.THERM_SENSOR_DS18B20, "011445ff9daa")
murotemp = muro.get_temperature()
try:
 while True:
 if murotemp > 20:
 ventein
 print(murotemp)
 print("Temperatur ist hoeher als 20 Grad, schalte ein...")
 time.sleep(0.5)
 muro = W1ThermSensor(W1ThermSensor.THERM_SENSOR_DS18B20, "011445ff9daa")
 murotemp = muro.get_temperature()
 elif murotemp < 20:
 ventaus
 print(murotemp)
 print("Temperatur ist niedriger als 20 Grad, schalte aus...")
 time.sleep(0.5)
 muro = W1ThermSensor(W1ThermSensor.THERM_SENSOR_DS18B20, "011445ff9daa")
 murotemp = muro.get_temperature()
except KeyboardInterrupt:
 GPIO.cleanup()
 print("exiting...")

• Ephemeral: First, you do nothing when murotemp == 20 it's normal ?
• Viktor Gruber: Yes, probably I should make it "if murotemp => 20: "
• Ephemeral: And, ventein = GPIO.output(20, 1) ventaus = GPIO.output(20, 0) are not in the loop in a conditionnal scope or other... then you init the same GPIO number at HIGH and then at LOW state for the global scope. That it's a logic problem because you use the same pin for ventein and ventaus
• Ephemeral: Yes if you want to check at 20 Grad
• Viktor Gruber: ok, now that I removed the global ventein = GPIO.output(20, 1) ventaus = GPIO.output(20, 0) it works!

Answer 1

You need to replace ventein with GPIO.output(20, 1) and ventaus with GPIO.output(20, 0).

You seem to think you have declared them as functions - you have not.

• Viktor Gruber: Thanks! That was the problem. (you are probably rightly suspecting it, I'm a bloody beginner, haha.)
• joan: I hope you will be pleased to know this is a common mistake and one which I think is quite logical to make. It's like assigning a macro in other languages.

Answer 2

As joan's answer has already pointed out the misunderstanding here is likely how to define and use a function in Python. Joan's approach will sure solve the immediate problem and make the program work. I will however address the underlying issue that is not so much related to the Pi but general Python programming.

So how to go about defining and using a function, from here:

• Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).
• Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.
• The first statement of a function can be an optional statement - the documentation string of the function or docstring.
• The code block within every function starts with a colon (:) and is indented.
• The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

So instead of

ventein = GPIO.output(20, 1)

this is what you're looking for:

def ventein():
 GPIO.output(20, 1)

Note the def keyword, the function name (ventein), the parentheses (empty in this case as no arguments are required), and the colon. The final return is ommitted in this case as it is optional if you do not want to pass anything back to the caller.

And when calling this new function instead of:

try:
 while True:
 if murotemp > 20:
 ventein

it should read:

try:
 while True:
 if murotemp > 20:
 ventein()