mirror of
https://github.com/KevinMidboe/python-gpiozero.git
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162 lines
4.9 KiB
ReStructuredText
162 lines
4.9 KiB
ReStructuredText
.. _faq:
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==========================
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Frequently Asked Questions
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==========================
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.. currentmodule:: gpiozero
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.. _keep-your-script-running:
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How do I keep my script running?
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================================
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The following script looks like it should turn an LED on::
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from gpiozero import LED
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led = LED(17)
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led.on()
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And it does, if you're using the Python (or IPython or IDLE) shell. However,
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if you saved this script as a Python file and ran it, it would flash on
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briefly, then the script would end and it would turn off.
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The following file includes an intentional :func:`~signal.pause` to keep the
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script alive::
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from gpiozero import LED
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from signal import pause
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led = LED(17)
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led.on()
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pause()
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Now the script will stay running, leaving the LED on, until it is terminated
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manually (e.g. by pressing Ctrl+C). Similarly, when setting up callbacks on
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button presses or other input devices, the script needs to be running for the
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events to be detected::
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from gpiozero import Button
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from signal import pause
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def hello():
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print("Hello")
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button = Button(2)
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button.when_pressed = hello
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pause()
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My event handler isn't being called?
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====================================
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When assigning event handlers, don't call the function you're assigning. For
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example::
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from gpiozero import Button
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def pushed():
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print("Don't push the button!")
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b = Button(17)
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b.when_pressed = pushed()
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In the case above, when assigning to ``when_pressed``, the thing that is
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assigned is the *result of calling* the ``pushed`` function. Because ``pushed``
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doesn't explicitly return anything, the result is ``None``. Hence this is
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equivalent to doing::
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b.when_pressed = None
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This doesn't raise an error because it's perfectly valid: it's what you assign
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when you don't want the event handler to do anything. Instead, you want to
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do the following::
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b.when_pressed = pushed
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This will assign the function to the event handler *without calling it*. This
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is the crucial difference between ``my_function`` (a reference to a function)
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and ``my_function()`` (the result of calling a function).
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Why do I get PinFactoryFallback warnings when I import gpiozero?
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================================================================
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You are most likely working in a virtual Python environment and have forgotten
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to install a pin driver library like ``RPi.GPIO``. GPIO Zero relies upon lower
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level pin drivers to handle interfacing to the GPIO pins on the Raspberry Pi,
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so you can eliminate the warning simply by installing GPIO Zero's first
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preference:
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.. code-block:: console
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$ pip install rpi.gpio
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When GPIO Zero is imported it attempts to find a pin driver by importing them
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in a preferred order (detailed in :doc:`api_pins`). If it fails to load its
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first preference (``RPi.GPIO``) it notifies you with a warning, then falls back
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to trying its second preference and so on. Eventually it will fall back all the
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way to the ``native`` implementation. This is a pure Python implementation
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built into GPIO Zero itself. While this will work for most things it's almost
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certainly not what you want (it doesn't support PWM, and it's quite slow at
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certain things).
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If you want to use a pin driver other than the default, and you want to
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suppress the warnings you've got a couple of options:
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1. Explicitly specify what pin driver you want via an environment variable. For
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example:
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.. code-block:: console
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$ GPIOZERO_PIN_FACTORY=pigpio python3
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In this case no warning is issued because there's no fallback; either the
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specified factory loads or it fails in which case an :exc:`ImportError` will
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be raised.
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2. Suppress the warnings and let the fallback mechanism work::
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>>> import warnings
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>>> warnings.simplefilter('ignore')
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>>> import gpiozero
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Refer to the :mod:`warnings` module documentation for more refined ways to
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filter out specific warning classes.
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How can I tell what version of gpiozero I have installed?
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=========================================================
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The gpiozero library relies on the setuptools package for installation
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services. You can use the setuptools ``pkg_resources`` API to query which
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version of gpiozero is available in your Python environment like so:
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.. code-block:: pycon
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>>> from pkg_resources import require
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>>> require('gpiozero')
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[gpiozero 1.4.0 (/usr/lib/python3/dist-packages)]
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>>> require('gpiozero')[0].version
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'1.4.0'
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If you have multiple versions installed (e.g. from ``pip`` and ``apt``) they
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will not show up in the list returned by the ``require`` method. However, the
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first entry in the list will be the version that ``import gpiozero`` will
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import.
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If you receive the error ``No module named pkg_resources``, you need to install
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``pip``. This can be done with the following command in Raspbian:
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.. code-block:: console
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$ sudo apt install python3-pip
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Alternatively, install pip with `get-pip`_.
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.. _get-pip: https://pip.pypa.io/en/stable/installing/
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