Design Guide

This guide covers a variety of development practices for CircuitPython core and library APIs. These APIs are both built-into CircuitPython and those that are distributed on GitHub and in the Adafruit and Community bundles. Consistency with these practices ensures that beginners can learn a pattern once and apply it throughout the CircuitPython ecosystem.

Start libraries with the cookiecutter

Cookiecutter is a tool that lets you bootstrap a new repo based on another repo. We’ve made one here for CircuitPython libraries that include configs for Travis CI and ReadTheDocs along with a setup.py, license, code of conduct and readme.

Cookiecutter will provide a series of prompts relating to the library and then create a new directory with all of the files. See the CircuitPython cookiecutter README for more details.

Module Naming

Adafruit funded libraries should be under the adafruit organization and have the format Adafruit_CircuitPython_<name> and have a corresponding adafruit_<name> directory (aka package) or adafruit_<name>.py file (aka module).

If the name would normally have a space, such as “Thermal Printer”, use an underscore instead (“Thermal_Printer”). This underscore will be used everywhere even when the separation between “adafruit” and “circuitpython” is done with a -. Use the underscore in the cookiecutter prompts.

Community created libraries should have the repo format CircuitPython_<name> and not have the adafruit_ module or package prefix.

Both should have the CircuitPython repository topic on GitHub.

Lifetime and ContextManagers

A driver should be initialized and ready to use after construction. If the device requires deinitialization, then provide it through deinit() and also provide __enter__ and __exit__ to create a context manager usable with with.

For example, a user can then use deinit()`:

import digitalio
import board

led = digitalio.DigitalInOut(board.D13)
led.direction = digitalio.Direction.OUTPUT

for i in range(10):
    led.value = True
    time.sleep(0.5)

    led.value = False
    time.sleep(0.5)
led.deinit()

This will deinit the underlying hardware at the end of the program as long as no exceptions occur.

Alternatively, using a with statement ensures that the hardware is deinitialized:

import digitalio
import board

with digitalio.DigitalInOut(board.D13) as led:
    led.direction = digitalio.Direction.OUTPUT

    for i in range(10):
        led.value = True
        time.sleep(0.5)

        led.value = False
        time.sleep(0.5)

Python’s with statement ensures that the deinit code is run regardless of whether the code within the with statement executes without exceptions.

For small programs like the examples this isn’t a major concern because all user usable hardware is reset after programs are run or the REPL is run. However, for more complex programs that may use hardware intermittently and may also handle exceptions on their own, deinitializing the hardware using a with statement will ensure hardware isn’t enabled longer than needed.

Verify your device

Whenever possible, make sure device you are talking to is the device you expect. If not, raise a RuntimeError. Beware that I2C addresses can be identical on different devices so read registers you know to make sure they match your expectation. Validating this upfront will help catch mistakes.

Getters/Setters

When designing a driver for a device, use properties for device state and use methods for sequences of abstract actions that the device performs. State is a property of the device as a whole that exists regardless of what the code is doing. This includes things like temperature, time, sound, light and the state of a switch. For a more complete list see the sensor properties bullet below.

Another way to separate state from actions is that state is usually something the user can sense themselves by sight or feel for example. Actions are something the user can watch. The device does this and then this.

Making this separation clear to the user will help beginners understand when to use what.

Here is more info on properties from Python.

Design for compatibility with CPython

CircuitPython is aimed to be one’s first experience with code. It will be the first step into the world of hardware and software. To ease one’s exploration out from this first step, make sure that functionality shared with CPython shares the same API. It doesn’t need to be the full API it can be a subset. However, do not add non-CPython APIs to the same modules. Instead, use separate non-CPython modules to add extra functionality. By distinguishing API boundaries at modules you increase the likelihood that incorrect expectations are found on import and not randomly during runtime.

When adding a new module for additional functionality related to a CPython module do NOT simply prefix it with u. This is not a large enough differentiation from CPython. This is the MicroPython convention and they use u* modules interchangeably with the CPython name. This is confusing. Instead, think up a new name that is related to the extra functionality you are adding.

For example, storage mounting and unmounting related functions were moved from uos into a new storage module. Terminal related functions were moved into multiterminal. These names better match their functionality and do not conflict with CPython names. Make sure to check that you don’t conflict with CPython libraries too. That way we can port the API to CPython in the future.

Example

When adding extra functionality to CircuitPython to mimic what a normal operating system would do, either copy an existing CPython API (for example file writing) or create a separate module to achieve what you want. For example, mounting and unmount drives is not a part of CPython so it should be done in a module, such as a new storage module, that is only available in CircuitPython. That way when someone moves the code to CPython they know what parts need to be adapted.

Document inline

Whenever possible, document your code right next to the code that implements it. This makes it more likely to stay up to date with the implementation itself. Use Sphinx’s automodule to format these all nicely in ReadTheDocs. The cookiecutter helps set these up.

Use Sphinx flavor rST for markup.

Lots of documentation is a good thing but it can take a lot of space. To minimize the space used on disk and on load, distribute the library as both .py and .mpy, MicroPython and CircuitPython’s bytecode format that omits comments.

Module description

After the license comment:

"""
`<module name>` - <Short description>
=================================================
<Longer description.>
"""

Class description

At the class level document what class does and how to initialize it:

class DS3231:
    """DS3231 real-time clock.

       :param ~busio.I2C i2c_bus: The I2C bus the DS3231 is connected to.
       :param int address: The I2C address of the device.
    """

    def __init__(self, i2c_bus, address=0x40):
        self._i2c = i2c_bus

Renders as:

class DS3231(i2c_bus, address=64)

DS3231 real-time clock.

Parameters:
  • i2c_bus (I2C) – The I2C bus the DS3231 is connected to.
  • address (int) – The I2C address of the device.

Attributes

Attributes are state on objects. (See Getters/Setters above for more discussion about when to use them.) They can be defined internally in a number of different ways. Each approach is enumerated below with an explanation of where the comment goes.

Regardless of how the attribute is implemented, it should have a short description of what state it represents including the type, possible values and/or units. It should be marked as (read-only) or (write-only) at the end of the first line for attributes that are not both readable and writable.

Instance attributes

Comment comes from after the assignment:

def __init__(self, drive_mode):
    self.drive_mode = drive_mode
    """
    The pin drive mode. One of:

    - `digitalio.DriveMode.PUSH_PULL`
    - `digitalio.DriveMode.OPEN_DRAIN`
    """

Renders as:

drive_mode

The pin drive mode. One of:

Property description

Comment comes from the getter:

@property
def datetime(self):
    """The current date and time as a `time.struct_time`."""
    return self.datetime_register

@datetime.setter
def datetime(self, value):
    pass

Renders as:

datetime

The current date and time as a time.struct_time.

Read-only example:

@property
def temperature(self):
    """
    The current temperature in degrees Celsius. (read-only)

    The device may require calibration to get accurate readings.
    """
    return self._read(TEMPERATURE)

Renders as:

temperature

The current temperature in degrees Celsius. (read-only)

The device may require calibration to get accurate readings.

Data descriptor description

Comment is after the definition:

lost_power = i2c_bit.RWBit(0x0f, 7)
"""True if the device has lost power since the time was set."""

Renders as:

lost_power

True if the device has lost power since the time was set.

Method description

First line after the method definition:

def turn_right(self, degrees):
    """Turns the bot ``degrees`` right.

       :param float degrees: Degrees to turn right
    """

Renders as:

turn_right(degrees)

Turns the bot degrees right.

Parameters:degrees (float) – Degrees to turn right

Use BusDevice

BusDevice is an awesome foundational library that manages talking on a shared I2C or SPI device for you. The devices manage locking which ensures that a transfer is done as a single unit despite CircuitPython internals and, in the future, other Python threads. For I2C, the device also manages the device address. The SPI device, manages baudrate settings, chip select line and extra post-transaction clock cycles.

I2C Example

from adafruit_bus_device import i2c_device

DEVICE_DEFAULT_I2C_ADDR = 0x42

class Widget:
    """A generic widget."""

    def __init__(self, i2c, address=DEVICE_DEFAULT_I2C_ADDR):
        self.i2c_device = i2c_device.I2CDevice(i2c, address)
        self.buf = bytearray(1)

    @property
    def register(self):
        """Widget's one register."""
        with self.i2c_device as i2c:
            i2c.writeto(b'0x00')
            i2c.readfrom_into(self.buf)
        return self.buf[0]

SPI Example

from adafruit_bus_device import spi_device

class SPIWidget:
    """A generic widget with a weird baudrate."""

    def __init__(self, spi, chip_select):
        # chip_select is a pin reference such as board.D10.
        self.spi_device = spi_device.SPIDevice(spi, chip_select, baudrate=12345)
        self.buf = bytearray(1)

    @property
    def register(self):
        """Widget's one register."""
        with self.spi_device as spi:
            spi.write(b'0x00')
            i2c.readinto(self.buf)
        return self.buf[0]

Use composition

When writing a driver, take in objects that provide the functionality you need rather than taking their arguments and constructing them yourself or subclassing a parent class with functionality. This technique is known as composition and leads to code that is more flexible and testable than traditional inheritance.

See also

Wikipedia has more information on “dependency inversion”.

For example, if you are writing a driver for an I2C device, then take in an I2C object instead of the pins themselves. This allows the calling code to provide any object with the appropriate methods such as an I2C expansion board.

Another example is to expect a DigitalInOut for a pin to toggle instead of a Pin from board. Taking in the Pin object alone would limit the driver to pins on the actual microcontroller instead of pins provided by another driver such as an IO expander.

Lots of small modules

CircuitPython boards tend to have a small amount of internal flash and a small amount of ram but large amounts of external flash for the file system. So, create many small libraries that can be loaded as needed instead of one large file that does everything.

Speed second

Speed isn’t as important as API clarity and code size. So, prefer simple APIs like properties for state even if it sacrifices a bit of speed.

Avoid allocations in drivers

Although Python doesn’t require managing memory, its still a good practice for library writers to think about memory allocations. Avoid them in drivers if you can because you never know how much something will be called. Fewer allocations means less time spent cleaning up. So, where you can, prefer bytearray buffers that are created in __init__ and used throughout the object with methods that read or write into the buffer instead of creating new objects. Unified hardware API classes such as busio.SPI are design to read and write to subsections of buffers.

Its ok to allocate an object to return to the user. Just beware of causing more than one allocation per call due to internal logic.

However, this is a memory tradeoff so do not do it for large or rarely used buffers.

Examples

struct.pack

Use struct.pack_into instead of struct.pack.

Sensor properties and units

The Adafruit Unified Sensor Driver Arduino library has a great list of measurements and their units. Use the same ones including the property name itself so that drivers can be used interchangeably when they have the same properties.

Property name Python type Units
acceleration (float, float, float) x, y, z meter per second per second
magnetic (float, float, float) x, y, z micro-Tesla (uT)
orientation (float, float, float) x, y, z degrees
gyro (float, float, float) x, y, z radians per second
temperature float degrees centigrade
eCO2 float equivalent CO2 in ppm
TVOC float Total Volatile Organic Compounds in ppb
distance float centimeters
light float non-unit-specific light levels (should be monotonic but is not lux)
lux float SI lux
pressure float hectopascal (hPa)
relative_humidity float percent
current float milliamps (mA)
voltage float volts (V)
color int RGB, eight bits per channel (0xff0000 is red)
alarm (time.struct, str) Sample alarm time and string to characterize frequency such as “hourly”
datetime time.struct date and time
duty_cycle int 16-bit PWM duty cycle (regardless of output resolution)
frequency int Hertz
value bool Digital logic
value int 16-bit Analog value, unit-less

Adding native modules

The Python API for a new module should be defined and documented in shared-bindings and define an underlying C API. If the implementation is port-agnostic or relies on underlying APIs of another module, the code should live in shared-module. If it is port specific then it should live in common-hal within the port’s folder. In either case, the file and folder structure should mimic the structure in shared-bindings.

To test your native modules or core enhancements, follow these Adafruit Learning Guides for building local firmware to flash onto your device(s):

SAMD21 - Build Firmware Learning Guide

ESP8266 - Build Firmware Learning Guide

MicroPython compatibility

Keeping compatibility with MicroPython isn’t a high priority. It should be done when its not in conflict with any of the above goals.