So you want to test ABC's with pytest
I was working on writing a new code challenge for Codechalleng.es the other day. It's based on inheritance and composition and uses Abstract Base Classes to define the interfaces that should be implemented in classes derived from it.
I was just finishing up with the tests and everything was passing.
Then when I checked the code coverage on it, I saw that it was complaining about my Site class, which is the one derived from ABC has several methods decorated as abstractmethods.
Coverage wanted me to test the pass statements on them...
Initial searches didn't turn up much, so I posed the question in PyBites slack channel. The best suggestion was to use the dis module and test it that way, but since it's not really running the code, coverage still counts it as untested.
How to test abstract methods
Finally I came across this post on reddit that had the solution. Here is what the code looks like, with doctrings removed:
class Site(ABC):
web: Web
def find_table(self, loc: int = 0) -> str:
return self.web.soup.find_all("table")[loc]
@abstractmethod
def parse_rows(self, table: Soup) -> List[Any]:
pass
@abstractmethod
def polls(self, table: int = 0) -> List[Any]:
pass
@abstractmethod
def stats(self, loc: int = 0):
pass
Here is how you go about testing them:
def test_site(test_file):
Site.__abstractmethods__ = set()
@dataclass
class Dummy(Site):
web: Web
url = "https://projects.fivethirtyeight.com/polls/"
test_web = Web(url, test_file)
d = Dummy(test_web)
table = d.find_table()
rows = d.parse_rows(table)
polls = d.polls()
stats = d.stats()
assert d.web.file.name == "test.html"
assert isinstance(Site, ABCMeta)
assert rows is None
assert polls is None
assert stats is None
As you can see, the way to accomplish this is to override __abstractmethods__.
Basically tricking it into thinking that it doesn't have any.
The last three asserts then verify that None is being returned.
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-4.4.0, py-1.8.0, pluggy-0.9.0 -- /home/mohh/anaconda3/envs/pybites/bin/python
cachedir: .pytest_cache
rootdir: /home/mohh/Projects/team-charlie/020
plugins: cov-2.8.1, asyncio-0.10.0
collected 8 items
test_composition.py::test_file_class PASSED [ 12%]
test_composition.py::test_web PASSED [ 25%]
test_composition.py::test_web_bad_url PASSED [ 37%]
test_composition.py::test_poll PASSED [ 50%]
test_composition.py::test_leaderboard PASSED [ 62%]
test_composition.py::test_rcp_stats PASSED [ 75%]
test_composition.py::test_nyt PASSED [ 87%]
test_composition.py::test_site PASSED [100%]
----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name Stmts Miss Cover Missing
----------------------------------------------
composition.py 107 0 100%
=========================== 8 passed in 7.22 seconds ===========================
Success!
Conclusion
If you're ever faced with having to write test for abstract methods, simply override the __abstractmethods__ of the ABC class!
Definitely something to keep in your toolbox.
Until next time, happy coding!