Example test structure

foo__slash__foo.py

class Foo(object):
    def bar(self):
        pass

    def baz(self):
        pass

tests__slash__foo_test.py

from pytest import raises

from foo import Foo

class TestBar(object):
    def test_returns_a_value_you_expect(self):
        assert Foo().bar() == some_value

    def test_raises_if_some_condition(self):
        do_something_that_sets_up_condition()
        with raises(SomeException):
            Foo().bar()

class TestBaz(object):
    def test_shit_happens_when_you_call_it(self):
        Foo().baz()
        assert something