Over the past week I went through some Python code in Stackstorm (hosted in GitHub ), and by doing so I learned a few cool new things. Maybe ‘new’ is not the right term, but these things were new to me and I wanted to share them with you.

The code I am referring to resides in a few places in Stackstorm, the first item:

@six.add_metaclass(abc.ABCMeta) class Access(object):

Two things here:

1. six.add_metaclass – makes Access a metaclass in a way which is compatible both with Python2 and Python3. In general, six package has a purpose of helping developers migrate their code from Python 2 to Python 3 by writing code which is compatible with both.

Hence the name: six = 2 * 3. To read more: https://pypi.python.org/pypi/six

2. abc.ABCMeta – abc is another builtin package, its purpose is to provide support for Abstract Base Classes. By making a class abstract you ensure that nobody can instantiate objects out of it.

This is useful for (at least) three purposes:

a. Maintain logic which is common to multiple classes in one place

@abc.abstractmethod (you can also force the inheriting classes to create specific class members by using the annotation @abc.abstractproperty). For more info in regards see b. force the users to implement methods which are annotated by(you can also force the inheriting classes to create specific class members by using the annotation). For more info in regards see the docs

not implement the abstractmethod or abstractproperty. By not implementing the abstract-methods/properties the inheriting class will become abstract as well. This technique is used in a few different places in StackStorm, if you want to see some examples, search for classes that inherit from c. creating a singleton. I’m still not convinced about the usefulness of creating a singleton in Python (because we have modules), but assuming you want to do so, you can either create a metaclass and call its methods as ClassName.method() or extend a metaclass butimplement the abstractmethod or abstractproperty. By not implementing the abstract-methods/properties the inheriting class will become abstract as well. This technique is used in a few different places in StackStorm, if you want to see some examples, search for classes that inherit from Access

3. This one took me a while to wrap my head around (requires a “functional programming” mind if you will):

def decorate(f): @functools.wraps(f) def callfunction(*args, **kwargs):

By calling @functools.wraps(f) we’re using a decorator inside a decorator. Not trivial…

To understand the main idea of ‘functools.wraps’, it’s easier to go through the following points/process (at least, easier for me…):

– When you decorate a function, you’re creating a new function that wraps the ‘original’ one, and return the wrapper

– The wrapper is used in order to be able to perform all kinds of tasks before/after the ‘original’ function is applied. Examples: preparing the arguments for the ‘original function’, timing how long does it take for the function to execute, add log printings before and after and etc.

– Say you have a wrapped function f, returned from a decorator, the user will then see the description (metadata) of the decorator instead of f, the original function.

– In order to avoid this confusion, @functools.wraps(f) comes into the picture: by annotating the wrapper function with @functools.wraps(f) – you’re making sure that the user of the wrapper will see the same description/metadata as the original function has. By ‘description’ I mean the function-signature (the name of the function and the names of the arguments it takes).

Hope this is helpful!