Raspberry Pi

Playing with Rainbow Hat I learned a few things about Python as a result I found out what a decorator is, the difference between args and kwargs and threads. I also learned that a lot of guides don’t understand either.

If you can’t explain it simply, you don’t understand it well enough.

Albert Einstein

Decorators

Rainbow Hat documentation says “use touch handlers either by passing in a function by name, or using the method as a decorator”.

Learning Python (Lutz, 2013) dedicates a chapter to decorators and sums it up rather well:

In short, decorators provide a way to insert automatically run code at the end of function and class definition statements—at the end of a def for function decorators, and at the end of a class for class decorators.

With similar notation to Java’s annotations:

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@decorator_function def function(arguments): ...

Python is running one function through another and binding the result to the original function name.

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def function(arguments):
    ...

function = decorator_function(function)

For example, Python has a built in function that returns a static method staticmethod(function). To make example_func static, we put:

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@staticmethod
def example_func(arg)
    ...

Which is rebound to:

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staticmethod(example_func)(arg)

So now I know what a decorator is in Python, I used it for the buttons. What to use them for though? I figure that they should control speed of LED, sequence or colour. That’s going to need a thread running as an event handler.

A short digression on arguments

What on earth is a key-worded argument? Lots of documentation refers to *args and **kwargs but had no idea what it was. Arguments passed to functions are read left to right:

function('Dougie', 42)

But we can also use a key-value pair:

function(name='Dougie', age=42)

Apart from improving readability in the function call, default arguments can be assigned in the function definition:

def function(name='Dougie', age=42)

By convention these are referred to as arg and kwarg. Almost there – that just leaves the *. Python lets you define functions that take any number of arguments, assembling them into a tuple. If you use key-value arguments, it assembles a dictionary.

def function(**kwargs): {...}

Now the clever(er) bit because if you do the same on the function call, Python unpacks the argument into individual arguments (*arg) or key-value pairs (**kwarg).

function(**kwargs)

Back to the main thread

The Rainbow Hat has buttons so I want to use these to control rainbow speed. This seems suited to a thread running an event handler. The syntax for the thread library (hopefully explaining the digression) is:

thread.start_new_thread (function_name, (*args, **kwargs))

Concurrency in Python is probably a post in its own right. The CPython interpreter bytecode isn’t fully thread safe. There are different interpretations of what that means so I’ll use the Open University definition:

A class is considered thread-safe if its instances behave under concurrent method calls as if they were called sequentially.

M362 Developing concurrent distributed systems (THE OU, 2008)

Python source code is compiled to bytecode which is run in a VM as machine code. In order to ensure only one thread executes bytecode at once the current thread has to hold a global lock (Global Interpreter Lock (GIL)).

This means multiple processor cores aren’t being used. In this application it doesn’t matter because the interpreter emulates concurrency by routinely switching threads.