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Automat
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Self-service finite-state machines for the programmer on the go.
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Automat is a library for concise, idiomatic Python expression of
finite-state
automata (particularly deterministic finite-state transducers).
Read more here, or on [Read the Docs]\ , or watch the following videos for
an overview and presentation
Overview and presentation by **Glyph Lefkowitz** at the first talk of the
first Pyninsula meetup, on February 21st, 2017:
:alt: Glyph Lefkowitz - Automat - Pyninsula #0
Presentation by **Clinton Roy** at PyCon Australia, on August 6th 2017:
:alt: Clinton Roy - State Machines - Pycon Australia 2017
Why use state machines?
^^^^^^^^^^^^^^^^^^^^^^^
Sometimes you have to create an object whose behavior varies with its
state,
but still wishes to present a consistent interface to its callers.
For example, let's say you're writing the software for a coffee machine.
It
has a lid that can be opened or closed, a chamber for water, a chamber for
coffee beans, and a button for "brew".
There are a number of possible states for the coffee machine. It might or
might not have water. It might or might not have beans. The lid might be
open
or closed. The "brew" button should only actually attempt to brew coffee
in
one of these configurations, and the "open lid" button should only work if
the
coffee is not, in fact, brewing.
With diligence and attention to detail, you can implement this correctly
using
a collection of attributes on an object; has_water\ , has_beans\ ,
is_lid_open and so on. However, you have to keep all these attributes
consistent. As the coffee maker becomes more complex - perhaps you add an
additional chamber for flavorings so you can make hazelnut coffee, for
example - you have to keep adding more and more checks and more and more
reasoning about which combinations of states are allowed.
Rather than adding tedious 'if' checks to every single method to make sure
that
each of these flags are exactly what you expect, you can use a state
machine to
ensure that if your code runs at all, it will be run with all the required
values initialized, because they have to be called in the order you declare
them.
You can read about state machines and their advantages for Python
programmers
in considerably more detail
[in this excellent series of articles from ClusterHQ].
What makes Automat different?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
There are
[dozens of libraries on PyPI implementing state machines].
So it behooves me to say why yet another one would be a good idea.
Automat is designed around this principle: while organizing your code
around
state machines is a good idea, your callers don't, and shouldn't have to,
care
that you've done so. In Python, the "input" to a stateful system is a
method
call; the "output" may be a method call, if you need to invoke a side
effect,
or a return value, if you are just performing a computation in memory.
Most
other state-machine libraries require you to explicitly create an input
object,
provide that object to a generic "input" method, and then receive results,
sometimes in terms of that library's interfaces and sometimes in terms of
classes you define yourself.
For example, a snippet of the coffee-machine example above might be
implemented
as follows in naive Python:
.. code-block:: python
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