Design by contract in python- part two
July 23, 2016
Last time I wrote about py.contracts- today is the time for another python library that helps to design by contracts.
This library is called zope.interface. From its GitHub page:
Interfaces are a mechanism for labeling objects as conforming to a given API or contract. So, this package can be considered as implementation of the Design By Contract methodology support in Python.
How is it translating to my example of ExternalAPIPort from the last
post? First I have to create interface for my port:
from zope.interface import Interface, implements
class IExternalAPIPort(Interface):
"""A port will perform search in external service."""
def search():
"""Perform search in external service."""
class ExternalAPIPort(object):
implements(IExternalAPIPort)
def __init__(self, adapter):
self.adapter = adapter
def search(self, query, *args, **kwargs):
return self.adapter.search(query, *args, **kwargs)Running this in python 3 will give you following traceback:
File "reddit-stars/external_api/external_api_port.py", line 11, in <module>
class ExternalAPIPort(object):
File "reddit-stars/external_api/external_api_port.py", line 12, in ExternalAPIPort
implements(IExternalAPIPort)
File ".virtualenvs/reddit-stars/lib/python3.5/site-packages/zope/interface/declarations.py", line 412, in implements
raise TypeError(_ADVICE_ERROR % 'implementer')
TypeError: Class advice impossible in Python3. Use the @implementer class decorator instead.So to use it with python 3 you have to:
from zope.interface import Interface, implementer
class IExternalAPIPort(Interface):
def search():
pass
@implementer(IExternalAPIPort)
class ExternalAPIPort(object):
def __init__(self, adapter):
self.adapter = adapter
def search(self, query, *args, **kwargs):
return self.adapter.search(query, *args, **kwargs)Thanks to that I have access to some checks that can help me with testing:
>>> IExternalAPIPort.implementedBy(ExternalAPIPort
True
>>> IExternalAPIPort.providedBy(instantiated_port)
TrueBut how to write a contract? It’s simple- let’s start by creating invariant:
def search_invariant(obj):
if not obj:
raise ValueError('query cannot be empty')
elif not isinstance(obj, str):
raise ValueError('query has to be string')
class IExternalAPIPort(Interface):
adapter = Attribute('external service')
def search():
passRight now when you run it, you will receive following error that query either has to be a string or not be empty:
File "/Development/reddit-stars/reddit_stars/urls.py", line 19, in <module>
views import RedditSearchVie
File "/Development/reddit-stars/search/views.py", line 4, in <module>
from .forms import RedditSearchForm
File "/Development/reddit-stars/search/forms.py", line 3, in <module>
from external_api.external_api_port import instantiated_port #, fake_port
File "/Development/reddit-stars/external_api/external_api_port.py", line 30, in <module>
IExternalAPIPort.validateInvariants(1)
File "/.virtualenvs/reddit-stars/lib/python3.5/site-packages/zope/interface/interface.py", line 438, in validateInvariants
call(obj)
File "/Development/reddit-stars/external_api/external_api_port.py", line 8, in search_invariant
raise ValueError('query has to be string')
ValueError: query has to be string
# or
ValueError: query cannot be emptyContract works! It assures that in running time that query will have to have values that are specified in a contract.
You can even declare your own invariant errors like this:
from zope.interface import Invalid
class QueryError(Invalid):
def __str__(self):
return "QueryError({})".format(*self.args)
def search_invariant(obj):
if not obj:
raise QueryError('query cannot be empty')
elif not isinstance(obj, str):
raise QueryError('query has to be string')And right now traceback will show custom error:
external_api.external_api_port.QueryError: QueryError(query cannot be empty)Unfortunately, I couldn’t find the way to declare contract for return
zope.intefrace. If you found one please let me know
That’s all for today! I hope you find this blog post interesting- feel free to comment. In the next blog post, I’ll write about another library in python for contracts and some tools from the standard library.
