9. Frequently Asked Questions

9.1. How do I keep my script running?

The following script looks like it should turn an LED on:

from gpiozero import LED

led = LED(17)
led.on()

And it does, if you’re using the Python (or IPython or IDLE) shell. However, if you saved this script as a Python file and ran it, it would flash on briefly, then the script would end and it would turn off.

The following file includes an intentional pause() to keep the script alive:

from gpiozero import LED
from signal import pause

led = LED(17)
led.on()

pause()

Now the script will stay running, leaving the LED on, until it is terminated manually (e.g. by pressing Ctrl+C). Similarly, when setting up callbacks on button presses or other input devices, the script needs to be running for the events to be detected:

from gpiozero import Button
from signal import pause

def hello():
    print("Hello")

button = Button(2)
button.when_pressed = hello

pause()

9.2. My event handler isn’t being called?

When assigning event handlers, don’t call the function you’re assigning. For example:

from gpiozero import Button

def pushed():
    print("Don't push the button!")

b = Button(17)
b.when_pressed = pushed()

In the case above, when assigning to when_pressed, the thing that is assigned is the result of calling the pushed function. Because pushed doesn’t explicitly return anything, the result is None. Hence this is equivalent to doing:

b.when_pressed = None

This doesn’t raise an error because it’s perfectly valid: it’s what you assign when you don’t want the event handler to do anything. Instead, you want to do the following:

b.when_pressed = pushed

This will assign the function to the event handler without calling it. This is the crucial difference between my_function (a reference to a function) and my_function() (the result of calling a function).

9.3. Why do I get PinFactoryFallback warnings when I import gpiozero?

You are most likely working in a virtual Python environment and have forgotten to install a pin driver library like RPi.GPIO. GPIO Zero relies upon lower level pin drivers to handle interfacing to the GPIO pins on the Raspberry Pi, so you can eliminate the warning simply by installing GPIO Zero’s first preference:

$ pip install rpi.gpio

When GPIO Zero is imported it attempts to find a pin driver by importing them in a preferred order (detailed in API - Pins). If it fails to load its first preference (RPi.GPIO) it notifies you with a warning, then falls back to trying its second preference and so on. Eventually it will fall back all the way to the native implementation. This is a pure Python implementation built into GPIO Zero itself. While this will work for most things it’s almost certainly not what you want (it doesn’t support PWM, and it’s quite slow at certain things).

If you want to use a pin driver other than the default, and you want to suppress the warnings you’ve got a couple of options:

  1. Explicitly specify what pin driver you want via an environment variable. For example:

    $ GPIOZERO_PIN_FACTORY=pigpio python3
    

    In this case no warning is issued because there’s no fallback; either the specified factory loads or it fails in which case an ImportError will be raised.

  2. Suppress the warnings and let the fallback mechanism work:

    >>> import warnings
    >>> warnings.simplefilter('ignore')
    >>> import gpiozero
    

    Refer to the warnings module documentation for more refined ways to filter out specific warning classes.

9.4. How can I tell what version of gpiozero I have installed?

The gpiozero library relies on the setuptools package for installation services. You can use the setuptools pkg_resources API to query which version of gpiozero is available in your Python environment like so:

>>> from pkg_resources import require
>>> require('gpiozero')
[gpiozero 1.4.1 (/usr/lib/python3/dist-packages)]
>>> require('gpiozero')[0].version
'1.4.1'

If you have multiple versions installed (e.g. from pip and apt) they will not show up in the list returned by the require method. However, the first entry in the list will be the version that import gpiozero will import.

If you receive the error No module named pkg_resources, you need to install pip. This can be done with the following command in Raspbian:

$ sudo apt install python3-pip

Alternatively, install pip with get-pip.