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pytest Goodies

Many pytest features were missing to make pytest_cases work with such a "no-boilerplate" experience. Many of these can be of interest to the general pytest audience, so they are exposed in the public API.


@fixture is similar to pytest.fixture but without its param and ids arguments. Instead, it is able to pick the parametrization from @pytest.mark.parametrize marks applied on fixtures. This makes it very intuitive for users to parametrize both their tests and fixtures. As a bonus, its name argument works even in old versions of pytest (which is not the case for fixture).

Finally it now supports unpacking, see unpacking feature.

@fixture deprecation if/when @pytest.fixture supports @pytest.mark.parametrize

The ability for pytest fixtures to support the @pytest.mark.parametrize annotation is a feature that clearly belongs to pytest scope, and has been requested already. It is therefore expected that @fixture will be deprecated in favor of @pytest_fixture if/when the pytest team decides to add the proposed feature. As always, deprecation will happen slowly across versions (at least two minor, or one major version update) so as for users to have the time to update their code bases.

unpack_fixture / unpack_into

In some cases fixtures return a tuple or a list of items. It is not easy to refer to a single of these items in a test or another fixture. With unpack_fixture you can easily do it:

import pytest
from pytest_cases import unpack_fixture, fixture

@pytest.mark.parametrize("o", ['hello', 'world'])
def c(o):
    return o, o[0]

a, b = unpack_fixture("a,b", c)

def test_function(a, b):
    assert a[0] == b

Note that you can also use the unpack_into= argument of @fixture to do the same thing:

import pytest
from pytest_cases import fixture

@pytest.mark.parametrize("o", ['hello', 'world'])
def c(o):
    return o, o[0]

def test_function(a, b):
    assert a[0] == b

And it is also available in fixture_union:

import pytest
from pytest_cases import fixture, fixture_union

@pytest.mark.parametrize("o", ['hello', 'world'])
def c(o):
    return o, o[0]

@pytest.mark.parametrize("o", ['yeepee', 'yay'])
def d(o):
    return o, o[0]

fixture_union("c_or_d", [c, d], unpack_into="a, b")

def test_function(a, b):
    assert a[0] == b


If you wish to share some parameters across several fixtures and tests, it might be convenient to have a fixture representing this parameter. This is relatively easy for single parameters, but a bit harder for parameter tuples.

The two utilities functions param_fixture (for a single parameter name) and param_fixtures (for a tuple of parameter names) handle the difficulty for you:

import pytest
from pytest_cases import param_fixtures, param_fixture

# create a single parameter fixture
my_parameter = param_fixture("my_parameter", [1, 2, 3, 4])

def fixture_uses_param(my_parameter):

def test_uses_param(my_parameter, fixture_uses_param):

# -----
# create a 2-tuple parameter fixture
arg1, arg2 = param_fixtures("arg1, arg2", [(1, 2), (3, 4)])

def fixture_uses_param2(arg2):

def test_uses_param2(arg1, arg2, fixture_uses_param2):

You can mark any of the argvalues with pytest.mark to pass a custom id or a custom "skip" or "fail" mark, just as you do in pytest. See pytest documentation.


As of pytest 5, it is not possible to create a "union" fixture, i.e. a parametrized fixture that would first take all the possible values of fixture A, then all possible values of fixture B, etc. Indeed all fixture dependencies (a.k.a. "closure") of each test node are grouped together, and if they have parameters a big "cross-product" of the parameters is done by pytest.

The topic has been largely discussed in pytest-dev#349 and a request for proposal has been finally made.

fixture_union is an implementation of this proposal. It is also used by @parametrize to support fixture_ref in parameter values, see below. The theory is presented in more details in this page, while below are more practical examples.

from pytest_cases import fixture, fixture_union

def first():
    return 'hello'

@fixture(params=['a', 'b'])
def second(request):
    return request.param

# c will first take all the values of 'first', then all of 'second'
c = fixture_union('c', [first, second])

def test_basic_union(c):


<...>::test_basic_union[\first] hello   PASSED
<...>::test_basic_union[\second-a] a    PASSED
<...>::test_basic_union[\second-b] b    PASSED


As you can see the ids of union fixtures are slightly different from standard ids, so that you can easily understand what is going on. You can change this feature with ìdstyle, see API documentation for details.

marks and ids

You can mark any of the alternatives with pytest.mark to pass a custom id or a custom "skip" or "fail" mark, just as you do in pytest. See pytest documentation.


Fixture unions also support unpacking with the unpack_into argument, see unpacking feature.

to conclude

Fixture unions are a major change in the internal pytest engine, as fixture closures (the ordered set of all fixtures required by a test node to run - directly or indirectly) now become trees where branches correspond to alternative paths taken in the "unions", and leafs are the alternative fixture closures. This feature has been tested in very complex cases (several union fixtures, fixtures that are not selected by a given union but that is requested by the test function, etc.). But if you find some strange behaviour don't hesitate to report it in the issues page !

IMPORTANT if you do not use @fixture but only @pytest.fixture, then you will see that your fixtures are called even when they are not used, with a parameter NOT_USED. This symbol is automatically ignored if you use @fixture, otherwise you have to handle it. Alternatively you can use @ignore_unused on your fixture function.

fixture unions vs. cases

If you're familiar with pytest-cases already, you might note that @cases_data is not so different than a fixture union: we do a union of all case functions. If one day union fixtures are directly supported by pytest, we will probably refactor this lib to align all the concepts.


@parametrize is a replacement for @pytest.mark.parametrize with many additional features to make the most of parametrization. See API reference for details about all the new features. In particular it allows you to include references to fixtures and to value-generating functions in the parameter values.

  • Simply use fixture_ref(<fixture>) in the parameter values, where <fixture> can be the fixture name or fixture function. New: from version 3.2 on, if auto_refs=True (default), @parametrize will automatically detect fixture symbols in the list of argvalues, and will create fixture_refs automatically around them so that you don't need to.
  • if you do not wish to create a fixture, you can also use lazy_value(<function>)
  • Note that when parametrizing several argnames, both fixture_ref and lazy_value can be used as the tuple, or in the tuple. Several fixture_ref and/or lazy_value can be used in the same tuple, too.
  • By default the id associated with a fixture_ref or a lazy_value is the name of the fixture or function. Custom ids can be passed with the id=<id> parameter.

For example, with a single argument:

import pytest
from pytest_cases import parametrize, fixture, fixture_ref, lazy_value

def world_str():
    return 'world'

def whatfun():
    return 'what'

@parametrize('who', [world_str, 'you'])
def greetings(who):
    return 'hello ' + who

@parametrize('main_msg', ['nothing', 
@pytest.mark.parametrize('ending', ['?', '!'])
def test_prints(main_msg, ending):
    print(main_msg + ending)

yields the following

> pytest -s -v
collected 12 items
test_prints[nothing-?] PASSED                   [  8%]nothing?
test_prints[nothing-!] PASSED                   [ 16%]nothing!
test_prints[world_str-?] PASSED                 [ 25%]world?
test_prints[world_str-!] PASSED                 [ 33%]world!
test_prints[whatfun-?] PASSED                   [ 41%]what?
test_prints[whatfun-!] PASSED                   [ 50%]what!
test_prints[1-?] PASSED                         [ 58%]1?
test_prints[1-!] PASSED                         [ 66%]1!
test_prints[greetings-world_str-?] PASSED       [ 75%]hello world?
test_prints[greetings-world_str-!] PASSED       [ 83%]hello world!
test_prints[greetings-you-?] PASSED             [ 91%]hello you?
test_prints[greetings-you-!] PASSED             [100%]hello you!

ids and marks

You can also mark any of the argvalues with pytest.param to pass a custom id or a custom "skip" or "fail" mark, just as you do in pytest. See pytest documentation.

You can also pass a custom callable or generator in ids as in @pytest.mark.parametrize.

idstyle customization

As you can see in the example above, the default ids are similar to what you would intuitively expect, even when you use fixture_ref.

This is because by default idstyle=None, to preserve test ids very close to standard pytest by default. But still, a fixture_union is generated behind the scenes when there is a fixture reference. So this is actually non-standard. You may therefore prefer to see explicit ids showing the various fixture alternatives, as in fixture_union. For this simply set the idstyle to 'compact', 'explicit' or to a callable such as str.

For example, changing the previous example to add idstyle="explicit":

(...same as above...)

@parametrize('main_msg', ['nothing',
                          fixture_ref(greetings)], idstyle="explicit")
@pytest.mark.parametrize('ending', ['?', '!'])
def test_prints(main_msg, ending):
    print(main_msg + ending)

yields to

> pytest -s -v
collected 12 items
test_prints[main_msg\nothing-?] PASSED                [  8%]nothing?
test_prints[main_msg\nothing-!] PASSED                [ 16%]nothing!
test_prints[main_msg\world_str-?] PASSED              [ 25%]world?
test_prints[main_msg\world_str-!] PASSED              [ 33%]world!
test_prints[main_msg\P2:4-whatfun-?] PASSED           [ 41%]what?
test_prints[main_msg\P2:4-whatfun-!] PASSED           [ 50%]what!
test_prints[main_msg\P2:4-1-?] PASSED                 [ 58%]1?
test_prints[main_msg\P2:4-1-!] PASSED                 [ 66%]1!
test_prints[main_msg\greetings-world_str-?] PASSED    [ 75%]hello world?
test_prints[main_msg\greetings-world_str-!] PASSED    [ 83%]hello world!
test_prints[main_msg\greetings-you-?] PASSED          [ 91%]hello you?
test_prints[main_msg\greetings-you-!] PASSED          [100%]hello you!

You can see that with this explicit style, the various "alternatives" in the fixture union generated behind the scenes for the main_msg parameter appear explicitly. In particular you see that there is an alternative main_msg\P2:4 covering several parameters in a row.

Note that this idstyle is not taken into account if you only use lazy_values but no fixture_ref, as lazy_values do not require to create a fixture union behind the scenes.

parametrization order

Another consequence of using fixture_ref is that the priority order of the parameters, relative to other standard pytest.mark.parametrize parameters that you would place on the same function, will get impacted. You may solve this by replacing your other @pytest.mark.parametrize calls with param_fixtures so that all the parameters are fixtures (see param_fixture).

passing a hook

As per version 1.14, all the above functions now support passing a hook argument. This argument should be a callable. It will be called everytime a fixture is about to be created by pytest_cases on your behalf. The fixture function is passed as the argument of the hook, and the hook should return it as the result.

You can use this fixture to better understand which fixtures are created behind the scenes, and also to decorate the fixture functions before they are created. For example you can use hook=saved_fixture (from pytest-harvest) in order to save the created fixtures in the fixture store.


assert_exception context manager is an alternative to pytest.raises to check exceptions in your tests. You can either check type, instance equality, repr string pattern, or use custom validation functions. See API reference.


pytest postprocesses the order of the collected items in order to optimize setup/teardown of session, module and class fixtures. This optimization algorithm happens at the pytest_collection_modifyitems stage, and is still under improvement, as can be seen in pytest#3551, pytest#3393, #2846...

Besides other plugins such as pytest-reorder can modify the order as well.

This new commandline is a goodie to change the reordering:

  • --with-reorder normal is the default behaviour: it lets pytest and all the plugins execute their reordering in each of their pytest_collection_modifyitems hooks, and simply does not interact

  • --with-reorder skip allows you to restore the original order that was active before pytest_collection_modifyitems was initially called, thus not taking into account any reordering done by pytest or by any of its plugins.