pytest fixture params with monkeypatch

Question

I am monkey patching a database connection class as a fixture. What I want to accomplish is to have some parameters passed to my monkey patched fixture since different tests will all require the same monkeypatch but with different return values.

@pytest.fixture
def data():
    now = datetime.datetime.now()
    data = Data(now)
    return data

@pytest.fixture
def patch_db(monkeypatch):
    class FakeDbConnection:
        def __init__(*args, **kwargs):
            pass

        def fetchall(self):
            return [1,2,3]

    monkeypatch.setattr(DbConnection, 'execute', FakeDbConnection)


def test_get_somevalue(patch_db, data):
    userids = data.get_userids()
    assert userids == [1,2,3]

def test_get_something_else(patch_db, data):
    userids = data.get_userids()
    assert userids == [6,7,8]

The problem I am having is that since my fetchall function only returns [1,2,3]. I tried to have patch_db fixture take params:

@pytest.fixture(scope='module', params=[[1, 2, 3], [4, 5, 6]])
def patch_db(monkeypatch, test_values):
    class FakeDbConnection:
        def __init__(*args, **kwargs):
            pass

        def fetchall(self):
            return test_values.param

    monkeypatch.setattr(DbConnection, 'execute', FakeDbConnection)

but I get the following errors: ScopeMismatch: You tried to access the 'function' scoped fixture 'monkeypatch' with a 'module' scoped request object, involved factories

Answer 1

You can't use a function fixture from a module fixture, as the the message says.

If you change patch_db's scope to function it will run, but this approach won't work as you expect, because test_get_somevalue would receive two patch_db fixtures, one patched with [1, 2, 3] and the other with [4, 5, 6].

I suggest that you add a function to FakeDbConnection that lets tests set what they want fetchall to return instead. Here's the full example:

@pytest.fixture
def data():
    now = datetime.datetime.now()
    data = Data(now)
    return data


@pytest.fixture
def patch_db(monkeypatch):
    class FakeDbConnection:
        def __init__(*args, **kwargs):
            pass

        def fetchall(self):
            return fetch_result.values

    monkeypatch.setattr(DbConnection, 'execute', FakeDbConnection)

    class FetchResult:
        pass

    fetch_result = FetchResult()
    fetch_result.values = None
    return fetch_result


def test_get_somevalue(patch_db, data):
    patch_db.values = [1,2,3]
    userids = data.get_userids()
    assert userids == [1,2,3]


def test_get_something_else(patch_db, data):
    patch_db.values = [6,7,8]
    userids = data.get_userids()
    assert userids == [6,7,8]