I covered py.contracts and zope.interface, now it's time to write about abc module from python standard library.
I wanted to write about another library dpcontrancts, but unfortunately, I wasn't able to download it from PyPi.
When I was reading and reviewing material for previous posts I found out that there is a way to use python standard library abc.ABCMeta for contracts- I decided to give a try.
First, you have to know what are metaclasses: understanding python metaclasses and what are they needed for: Why use Abstract Base Classes in Python?.
After this introduction let's jump straight to the code:
importabcclassExternalAPIPortMetaclass(object,metaclass=abc.ABCMeta):@abc.abstractmethoddefsearch(self,query,*args,**kwargs):ifnotisinstance(query,str):raiseValueError('Query should be string')classExternalAPIPort(ExternalAPIPortMetaclass):def__init__(self,adapter):self.adapter=adapterdefsearch(self,query,*args,**kwargs):super(ExternalAPIPort,self).search(query,*args,**kwargs)returnself.adapter.search(query,*args,**kwargs)What is happening here? I defined ExternalAPIPortMetaclass as a metaclass that inherits from abc.ABCMeta (This code snippet is valid for python 3). Then I decided to make abstractmethod called search so all instances of that metaclass will have to provide such function. Inside this code, I check whether provided query is a string or not. In ExternalAPIPort which inherits from previously defined I have to call super for ExternalAPIPortMetaclass search method. Thanks to that I can make a validation of query. Right after that I simply return search query.
What I don't like there is fact that I need to add additional line of code inside ExternalAPIPort.search with super just for checking contract which can trick others. That's why I think that metaclasses and contracts are two different topic besides that they have some pieces in common (both are designed for telling: here I make contract that you must obey).
To sum up this whole series I belive contracts are usefull for telling others that I made agrement that this function has to take and return certain value. In python word where is so called ducktyping and I don't think they are necessary in every case but designing by contracts can be helpfull as I shown in examples. In my day to day work I use contracts the same way to make agrements on ports methods in ports and adapters design pattern.
Thank you for reading! Feel free to comment!