Fixture functions using “yield” / context manager integration¶
New in version 2.4.
pytest-2.4 allows fixture functions to seamlessly use a
return statement to provide a fixture value while otherwise
fully supporting all other fixture features.
Let’s look at a simple standalone-example using the
# content of test_yield.py import pytest @pytest.yield_fixture def passwd(): print ("\nsetup before yield") f = open("/etc/passwd") yield f.readlines() print ("teardown after yield") f.close() def test_has_lines(passwd): print ("test called") assert passwd
In contrast to finalization through registering callbacks, our fixture function used a
statement to provide the lines of the
The code after the
yield statement serves as the teardown code,
avoiding the indirection of registering a teardown callback function.
Let’s run it with output capturing disabled:
$ py.test -q -s test_yield.py setup before yield test called .teardown after yield 1 passed in 0.12 seconds
We can also seamlessly use the new syntax with
Let’s simplify the above
# content of test_yield2.py import pytest @pytest.yield_fixture def passwd(): with open("/etc/passwd") as f: yield f.readlines() def test_has_lines(passwd): assert len(passwd) >= 1
f will be closed after the test finished execution
because the Python
file object supports finalization when
with statement ends.
Note that the yield fixture form supports all other fixture
features such as
params, etc., thus changing existing
fixture functions to use
yield is straightforward.
yield syntax is similar to what
contextlib.contextmanager() decorated functions
provide, with pytest fixture functions the part after the
“yield” will always be invoked, independently from the
exception status of the test function which uses the fixture.
This behaviour makes sense if you consider that many different
test functions might use a module or session scoped fixture.
Discussion and future considerations / feedback¶
There are some topics that are worth mentioning:
yieldis used for producing multiple values. But fixture functions can only yield exactly one value. Yielding a second fixture value will get you an error. It’s possible we can evolve pytest to allow for producing multiple values as an alternative to current parametrization. For now, you can just use the normal fixture parametrization mechanisms together with
yieldintroduces more than one way to write fixture functions, so what’s the obvious way to a newcomer?
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