py.test skips test class if constructor is defined

I have following unittest code running via py.test. Mere presence of the constructor make the entire class skip when running py.test -v -s

collected 0 items / 1 skipped

Can anyone please explain to me this behaviour of py.test?

I am interested in understanding py.test behaviour, I know the constructor is not needed.

Thanks, Zdenek

class TestClassName(object):
    def __init__(self):
       pass

    def setup_method(self, method):
       print "setup_method called"
    
    def teardown_method(self, method):
       print "teardown_method called"

    def test_a(self):
       print "test_a called"
       assert 1 == 1

    def test_b(self):
       print "test_b called"
       assert 1 == 1

The documentation for py.test says that py.test implements the following standard test discovery:

• collection starts from the initial command line arguments which may be directories, filenames or test ids. recurse into directories, unless they match norecursedirs
• test_*.py or *_test.py files, imported by their package name.
• Test prefixed test classes (without an __init__ method) [<-- notice this one here]
• test_ prefixed test functions or methods are test items

So it's not that the constructor isn't needed, py.test just ignores classes that have a constructor. There is also a guide for changing the standard test discovery.

As already mentioned in the answer by Matti Lyra py.test purposely skips classes which have a constructor. The reason for this is that classes are only used for structural reasons in py.test and do not have any inherent behaviour, while when actually writing code it is the opposite and much rarer to not have an .__init__() method for a class. So in practice skipping a class with a constructor will likely be what was desired, usually it is just a class which happens to have a conflicting name.

Lastly py.test needs to instantiate the class in order to execute the tests. If the constructor takes any arguments it can't instantiate it, so again skipping is the right thing to do.