Returning multiple objects from a pytest fixture

Question:

I am learning how to use pytest by testing a simple event emitter implementation.

Basically, it looks like this

class EventEmitter():
    def __init__(self):
        ...
    def subscribe(self, event_map):
        # adds listeners to provided in event_map events
    def emit(self, event, *args):
        # emits event with given args

For convenience, I created a Listener class that is used in tests

class Listener():
    def __init__(self):
        ...
    def operation(self):
        # actual listener

Currently, test looks the following way

@pytest.fixture
def event():
    ee = EventEmitter()
    lstr = Listener()
    ee.subscribe({"event" : [lstr.operation]})
    return lstr, ee

def test_emitter(event):
    lstr = event[0]
    ee = event[1]
    ee.emit("event")
    assert lstr.result == 7 # for example

In order to test event emitter, I need to check whether the inner state of the listener has changed after event propagation. Thus, I need two objects and I wonder if there is a better way to do this (maybe use two fixtures instead of one somehow) because this looks kinda ugly to me.

Asked By: Zallin

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Source

Answers:

You will probably need two fixtures in this case.

You can try the @pytest.yield_fixture like:

@pytest.yield_fixture
def event():
    ...
    yield <event_properties>

@pytest.yield_fixture
def listener(event):
    ...
    yield <listener_properties>

Note: this is now deprecated https://docs.pytest.org/en/latest/yieldfixture.html

Answered By: Ederson Badeca

Usually in order to avoid tuples and beautify your code, you can join them back together to one unit as a class, which has been done for you, using collections.namedtuple:

import collections
EventListener = collections.namedtuple('EventListener', 'event listener')

Now modify your fixture:

@pytest.fixture
def event_listener():
 e = EventListener(EventEmitter(), Listener())
 e.event.subscribe({'event' : [e.listener.operation]})
 return e

Now modify your test:

def test_emitter(event_listener):
 event_listener.event.emit('event')
 assert event_listener.listener.result == 7
Answered By: Sawel

You should use a Python feature called iterable unpacking into variables.

def test_emitter(event):
    lstr, ee = event # unpacking
    ee.emit("event")
    assert lstr.result == 7

Basically, you are assigning event[0] to lstr, and event[1] to ee. Using this feature is a very elegant way to avoid using indexes.

Discarding

In case you are going to use your fixture in mutiple tests, and you don’t need all values in every test, you can also discard some elements of the iterable if you are not interested in using them as follows:

l = ['a', 'b', 'c', 'd']
a, b, c, d = l # unpacking all elements
a, _, c, d = l # discarding b
a, _, _, d = l # python 2: discard b and c
a, *_, d = l # python 3: discard b and c
a, _, _, _ = l # python2: discard, b, c and d
a, *_ = l # python3: discard b, c, and d

In theory, you are not literally discarding the values, but in Python _, so-called “I don’t care”, is used for ignoring the specific values.

Answered By: lmiguelvargasf

If you can not afford to easily split your tuple fixture into two independent fixtures, you can now “unpack” a tuple or list fixture into other fixtures using my pytest-cases plugin as explained in this answer.

For your example that would look like:

from pytest_cases import pytest_fixture_plus

@pytest_fixture_plus(unpack_into="lstr,ee")
def event():
    ee = EventEmitter()
    lstr = Listener()
    ee.subscribe({"event" : [lstr.operation]})
    return lstr, ee

def test_emitter(lstr, ee):
    ee.emit("event")
    assert lstr.result == 7 # for example
Answered By: smarie

I landed here when searching for a similar topic.

Due to lack of reputation I cannot comment on the answer from @lmiguelvargasf (https://stackoverflow.com/a/56268344/2067635) so I need to create a separate answer.

I’d also prefer returning multiple values and unpacking them to individual variables. This results in concise, Pythonic code.

There is one caveat, though:

For tests that rely on fixtures with autouse=True, this results in a TypeError

Example:

@pytest.fixture(autouse=True)
def foo():
    return 1, 2

# Gets fixture automagically through autouse
def test_breaks():
    arg1, arg2 = foo
    assert arg1 <= arg2

# Explicit request for fixture foo
def test_works(foo):
    arg1, arg2 = foo
    assert arg1 <= arg2


test_breaks 
FAILED                    [100%]
def test_breaks():
>       arg1, arg2 = foo
E       TypeError: cannot unpack non-iterable function object


test_works 
=============== 1 passed in 0.43s ===============
Process finished with exit code 0
PASSED                     [100%]

The fix is easy. But it took me some time to figure out what the problem was, so I thought I share my findings.

Answered By: twil