A Python module that emulates the BBC microbit as defined by the microbit micropython API.
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A Python module that emulates the BBC microbit as defined by the microbit micropython API.

Any errors in the emulation are entirely due to errors in my program logic or in my understanding of the microbit module API and not the responsibility of the developers of the microbit micropython API.


This Python module is based on the microbit micropython API defined as part of the BBC microbit project.

See: https://github.com/bbcmicrobit/micropython for the microbit micropython project repository and licensing and https://microbit-micropython.readthedocs.io/en/latest/ for the microbit micropython documentation.

The aim of this project is to provide an implementation of the microbit micropython API that can be used to test a microbit micropython program without having to have a physcial microbit connected. This is similar to the in-browser microbit emulations for other programming languages provided at http://microbit.co.uk/.

If a student develops using the Web based Python editor, feedback with respect to bugs in their program is via a Python error message scrolling across the physical microbit screen. These are quite difficult to read and could become a barrier to development. Development in Python is likely to lead to simple syntax errors (indentation etc.) that only become apparent when an error message is displayed on the microbit screen. The hope is that it will be easier for students to find and correct bugs if they have more readable error output.

The offline mu micropython editor has a REPL shell that, if connected to a microbit, will display error output in the shell and support easier debugging. Even so, there may be some benefit in being able to trace and debug a program without connecting to the physical microbit.

This module supports disconnected program development by emulating most of the microbit's functionality (see the "What is emulated" section of this README for more details). Emulation output is text to the console, which means the module can be used in many programming environments. There is no attempt to do graphical emulation. Providing a graphical interface may be something for the future. It would have the advantage of providing an alternative mechanism for manipulating the emulated state (see the section of this README on "Emulating and changing state" for how this is currently done). As it is, the module allows testing of both the syntax and logic of microbit micropython programs.

The name of the module is microbit_stub to distinguish it from the official microbit module.

The above means that you can take a program written for the microbit module and simply change the relevant import statement to import microbit_stub. The program can then be tested in a standard Python development environment or at the console. Once the program has been tested it can be copied to the Web based or mu editor and uploaded to the microbit (after reinstating the microbit module import statement). For example, the following Python program developed in the browser or using mu:

from microbit import *
display.scroll('hello world!')

will scroll 'hello world!' across the microbit screen.

The following Python program using microbit_stub:

from microbit_stub import *
display.scroll('hello world!')

will output a text emulation of scrolling 'hello world!' in a console window (e.g. the IDLE shell or console of some other IDE or a terminal window).

Apart from the change of module name, the program runs unchanged in the different environments.

It should be safe to rename this module microbit and not cause any conflict with the Web based or mu editor. On balance, I thought it better to use a different name to distinguish between the two. If you change the name of the microbit_stub module to microbit (by renaming microbit_stub.py to microbit.py) then you will not need to change import statements when switching programming environments. The microbit subdirectory contains programs and related files that take this approach.


As shown above, to use the module simply replace the line:

from microbit import *


from microbit_stub import *

and run your pograms in IDLE or some other Python IDE or from the command line.

As with any other Python module, the microbit_stub.py file must be in your Python search path (this could be the same folder as the program(s) you are testing).

Documentation is included in the microbit_stub module file.

The distribution also includes:

test_microbit_stub.py (and test_microbit_stub_display.txt) - unittest and doctest tests of the module (test_microbit_stub.py runs all tests, including the doctests of the display)

happysad.py - an example program that displays Image.HAPPY on button A press and Image.SAD otherwise. This program will also work with the physical microbit.

bitcounter-range.py - an example program that uses the microbit display to count up to 31 on button A presses and convert the current binary number to decimal on a button B press. This program will also work with the physical microbit.

pressbutton.py - a program to simulate pressing and releasing a button. It takes the button name (button_a or button_b) as a command line argument or uses button_a by default. This is not a microbit program but can be used to test programs that react to button presses (see the next section of this README for more information). It could be adapted to simulate changes to the state of pins.

pressbutton_withreset.py - this is similar to pressbutton.py but resets the state of the microbit after 100 button presses and then exits. This is also for testing purposes.

The above programs should give some idea of how to use the microbit_stub module.

What is emulated

The module provides an implementation of the following parts of the microbit micropython API:

# global functions

# classes
Image   # including built-in images

The I2C and UART classes and methods are defined but the implementation of methods is simply to pass. Programs that use the I2C or UART classes should run (up to a point) but will not do anything interesting.

Internally, images are stored as a 5x5 list of lists of microbit pixel values. The values are in the range 0 to 9 corresponding to the microbit pixel intensity values.

The display of an image is a text border around 5 characters that represent each row of the microbit display. 0s are represented by a space and other pixel values by their intensity. For example, the following program:

from microbit_sbut import *


|     |
| 999 |
|9  9 |
|9  9 |
| 9999|

to the console. In output, spaces represent 0s to make the image clearer

When using display.show() to show a string of charactes, they are printed vertically to the console. E.g.:

from microbit_sbut import *


|     |
| 999 |
|9  9 |
|9  9 |
| 9999|
|9    |
|9    |
|999  |
|9  9 |
|999  |

As with the physical microbit, showing a string with runs of repeated characters results in superimposition. That is display.show('hello') superimposes the 2nd l on the first. Scrolling a string with repeated characters shows all characters.

In addition to the implementation of the microbit classes and global functions such as sleep, microbit_stub extends the API with a State class and a single state instance. This represents the state of buttons, pins, and accelerometer x, y and z values. For example, reading from a pin involves reading from a corresponding value of the state object. Writing to a pin, changes a corresponding value of the state object. See the next section for information on how to use the state instance to simulate state changes.

Accelerometer gestures are randomly generated, as are compass headings and field strengths. They are not stored with the state object. Current image state is maintained by the image instance and is not stored with the state object.

Emulating and changing microbit state (input/output)

The "state" of a physical microbit is determined by button presses, inputs and output to pins etc. The microbit_stub does not have these physical inputs and outputs. Instead, internally, the state of the emulated microbit is represented by a dictionary. In the normal case this state representation is loaded from and saved to one or more json files. This internal representation is managed by and manipulated through a state object.

microbit micropython programs can be tested with the microbit_stub module without any direct interaction with state object. The exposure of the state object, and underlying state files, simply opens up the possibility of more extensive testing and simulation of input/output events.

The initial state is:

    "accelerometer_x": 0,
    "accelerometer_y": 0,
    "accelerometer_z": 0,
    "button_a": 0,
    "button_a_presses": 0,
    "button_b": 0,
    "button_b_presses": 0,
    "pin0": 0,
    "pin1": 0,
    "pin2": 0,
    "pin3": 0,
    "pin4": 0,
    "pin5": 0,
    "pin6": 0,
    "pin7": 0,
    "pin8": 0,
    "pin9": 0,
    "pin10": 0,
    "pin11": 0,
    "pin12": 0,
    "pin13": 0,
    "pin14": 0,
    "pin15": 0,
    "pin16": 0,
    "pin19": 0,
    "pin20": 0,
    "power": 1,
    "state_file": "microbit_state.json"

There are entries for:

  • accelerometer x, y and z values,
  • buttons A and B (which will be 1 if currently pressed) and for counts of button presses,
  • available pins (0 to 16 and 19 and 20)

By default, the initial state of all of these values is 0 or off.

In addition, there are entries for:

  • microbit power. If power is 1, the microbit is on and display output will be printed. If power is 0, there is no output.
  • state_file - to specify where to persist microbit state and to allow chaining of state files to simulate state changes, e.g. by cycling through a series of state files. The state_file entry can be thought of as a pointer to the next state of the microbit, contained in the specified state_file.

The state object provides controlled access to the representation of state, with validity checking of state changes.

The initial state file is specified in the microbit_stub_settings.py config file. This settings file is just a module that the microbit_stub module imports. If microbit_stub_settings.py cannot be found, the default state file is microbit_state.json. If a state file cannot be found or opened, then in-memory representation of state is used and it is not possible to manipulate microbit state from multiple processes.

File input/output errors are silently ignored. That is, failure to open, read from or write to a file is ignored and the module reverts to in-memory operation. This is behaviour that could be configured with relatively minor changes to the module. However, I thought it better to simply revert to something that works in-memory in the event of file access failures. It does mean that users must be aware that incorrect filenames or non-existent files or files with incorrect permissions may result in their tests not behaving as expected.

State is dumped to file after any state change (including on reset).

State is loaded from file on initialisation of the module, on wake up from a sleep (to simulate events occuring during the passing of time), and before a state change is dumped (to include any intervening state changes by other processes).

There is no concurrency control on state file access because:

  1. File locking is notoriously difficult to do cross platform and, in any case, is error-prone
  2. It is unnecessary. Conflicting concurrent access is results in missed state changes. This is the same as missing a button press. Such missed updates are "normal" behaviour for a microbit.

The State.dump and State.load methods are the places to look if you with to change this behaviour.

There are essentially three ways to simulate state changes:

  • add state object method calls to a microbit program. This will work but has the disadvantage that the program is no longer a standard microbit progam and any code invoking state methods must be removed or commented out before uploading to the microbit
  • use separate program(s) to invoke state object methods. That is, run the microbit program and one or more state changing programs that all operate on the same underlaying state file(s). An example of this approach is to run the happysad.py program in one terminal and the pressbutton.py program in another terminal using the same configuration. The pressbutton program should cause the happysad program to alternate between happy and sad faces.
  • chain state files to simulate state changes. An example of this approach is to configure the initial state file to be microbit_state_00.json or microbit_state_01.json. In microbit_state_00.json, the value for button_a is 0 and the value for state_file is microbit_state_01.json. In microbit_state_01.json, the value for button_a is 1 and the value for state_file is microbit_state_00.json. Using this configuration has the effect of alternating between each state file and, therefore, alternating between button_a value 1 (pressed) and 0 (not pressed).

This section may appear complex. If it does, remember:

  1. program testing can be done without the above manipulations of state (it is additional functionality)
  2. the explanations may become clearer by inspecting and running the programs provide in the distribution (and by more documentation that may be provided at a later date, if I get time!)