Completing Your Stack Class & Measuring Test Coverage

Simply and clear. Nice course for pytest newbie.

I may have missed it somewhere in the course, but the cov plugin was not installed by default on my windows machine while working in venv. I had to do a little googling to solve the error I received:

docs.pytest.org/en/latest/plugins.html?highlight=cov

Hope this helps someone in the future.

Yes, definitively the “coverage” plugin is missing. We can simply install that by running:

pip install pytest-cov

Also, I don’t know why but all my “site-packages” files are getting returned by python -m pytest --cov. Does anyone know how to return only my project files as it’s returned on video lesson?

You can run the command.

python -m pytest -v --cov=ds

OR Create a .coveragerc file with following

[run]
source = ds

Your directory structure should look like this.

-rw-r--r-- 1 user user   17 Mar 20 12:14 .coveragerc
drwxr-xr-x 3 user user 4.0K Mar 20 11:25 ds/
drwxr-xr-x 3 user user 4.0K Mar 20 11:37 tests/

Nice! Thanks @Chris, @Rodrigo Vieira and @Isaac Dadzie for filling in the last missing pieces and pointing us forward here :)

Hello, why class constructor in push function don’t have return keyword like the others?

def push(self, item):
    self._storage.append(item)

Hi @Lokman,

Functions don’t aways have to return something. In this case, the push function only appends an item to _storage. We don’t need to have it tell us anything once it’s done. Unlike def __len__() where we want to know what the length is. Hence this returns the length of the current Stack to us.

Hope that helps.

Thanks @Ricky White for the answer.

This guy is great. A lot of times when watching videos, I have a question in my head that never gets answered. This guy guessed my questions, spoke them out loud and then answered them not once but twice.

Kudos for not leaving out important details.

Would like to see a pytest assertion that the proper error is thrown/raised.

It might have been helpful to demonstrate how to write a test for a call to a method that is expected to throw an exception. For example, instead of returning None from the pop method when the stack is empty (which we wouldn’t want to do if we actually want to be able to push None onto the stack) we might want to let the IndexError propagate instead of catching it, or perhaps raise our own custom StackUnderflowError.

Assuming we just want to let the IndexError propagate, one way to test what happens when the stack is empty (assuming the same fixture setup) would be something like this:

def test_pop_when_empty(stack):
  try:
    stack.pop()
    assert False
  except IndexError:
    assert True

Is there a more concise approach?

@Carl

From my understanding, something like this should be used for exception testing.

def test_pop_when_empty(stack):
    with pytest.raises(IndexError):
        stack.pop()

Or even something better 😉:

def test_pop_when_empty(stack):
    with pytest.raises(StackEmptyError):
        stack.pop()

since raising a custom exception in this case would be advisable. This custom exception can wrap the original one by using the raise ... from OriginalError construct.

The parameter stack passed to the test functions is confusing. stack is technically a fixture function which returns an instance of the class Stack.

This is where it gets confusing for me: .push() and .pop() methods are called on the fixture stack and not Stack

Can you help elaborate on the syntax?

Thanks.

You need to install pytest-cov then

@Chris It helped me buddy 👍 !!

I was going to ask about this error I got this from using the –cov argument (and now I don’t have to). ERROR: usage: main.py [options] [file_or_dir] [file_or_dir] […] main.py: error: unrecognized arguments: –cov inifile: None rootdir: C:\Users\<jchurchi (my user)>\dev\real_python\testing\ds