python - In which order are pytest fixtures executed?

Question

For an application I'm testing I'd like to create an autouse=True fixture which monkeypatches smtplib.SMTP.connect to fail tests if they try to send an email unexpectedly.

However, in cases where I do expect tests to send emails, I want to use a different fixture logging those emails instead (most likely by using the smtpserver fixture from pytest-localserver and monkeypatching the connect method to use the host/port returned by that fixture)

Of course that can only work if the autouse fixture is executed before the other fixture (loaded as funcarg). Is there any specific order in which fixtures are executed and/or is there a way to guarantee the execution order?

Answer 1

The easiest way to control the order in which fixtures are executed, is to just request the previous fixture in the later fixture. So to make sure b runs before a:

@pytest.fixture(autouse=True, scope="function")
def b():
    pass

@pytest.fixture(scope="function")
def a(b):
    pass

For details on the general fixture resolution order, see Maxim's excellent answer below or have a look at the documentation.