Using capsys in fixtures: AttributeError: 'CaptureFixture' object has no attribute '_capture'

[hroncok]

Recently, I had a workshop about parametric tests and fixtures in pytest. As one of the examples I tried to use the capsys fixture inside my own and failed. I wonder if it's a bug or documented behavior I couldn't find.

First, working code

def test_print(capsys):
    print(123, 456, 789)
    out, err = capsys.readouterr()
    assert not err.strip()
    assert '5' in out

$ python -m pytest demo.py 
... 1 passed in 0.01 seconds ...

Broken code

Now I don't like multiple assertions in one test, so let's create a fixture and split it:

import pytest

OUT = 0
ERR = 1


@pytest.fixture
def captured_print(capsys):
    print(123, 456, 789)
    out, err = capsys.readouterr()
    return out.strip(), err.strip()


def test_print_no_stderr(captured_print):
    assert not captured_print[ERR]


def test_print_stdout_contains_5(captured_print):
    assert '5' in captured_print[OUT]

I think this should work. At least, it feels like a natural way of doing it. But it blows up:

$ python -m pytest demo.py 
... AttributeError: 'CaptureFixture' object has no attribute '_capture' ...

Full output

============================= test session starts ==============================
platform linux -- Python 3.6.2, pytest-3.2.2.dev17+g539523cf, py-1.4.34, pluggy-0.4.0
rootdir: /home/churchyard/Dokumenty/pytest, inifile: tox.ini
collected 2 items                                                               

demo.py EE

==================================== ERRORS ====================================
____________________ ERROR at setup of test_print_no_stderr ____________________

self = <_pytest.capture.CaptureFixture object at 0x7fe560f30c50>

    def readouterr(self):
        try:
>           return self._capture.readouterr()
E           AttributeError: 'CaptureFixture' object has no attribute '_capture'

../_pytest/capture.py:208: AttributeError

During handling of the above exception, another exception occurred:

capsys = <_pytest.capture.CaptureFixture object at 0x7fe560f30c50>

    @pytest.fixture
    def captured_print(capsys):
        print(123, 456, 789)
>       out, err = capsys.readouterr()

demo.py:10: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <_pytest.capture.CaptureFixture object at 0x7fe560f30c50>

    def readouterr(self):
        try:
            return self._capture.readouterr()
        except AttributeError:
>           return self._outerr
E           AttributeError: 'CaptureFixture' object has no attribute '_outerr'

../_pytest/capture.py:210: AttributeError
---------------------------- Captured stdout setup -----------------------------
123 456 789
________________ ERROR at setup of test_print_stdout_contains_5 ________________

self = <_pytest.capture.CaptureFixture object at 0x7fe560f6a358>

    def readouterr(self):
        try:
>           return self._capture.readouterr()
E           AttributeError: 'CaptureFixture' object has no attribute '_capture'

../_pytest/capture.py:208: AttributeError

During handling of the above exception, another exception occurred:

capsys = <_pytest.capture.CaptureFixture object at 0x7fe560f6a358>

    @pytest.fixture
    def captured_print(capsys):
        print(123, 456, 789)
>       out, err = capsys.readouterr()

demo.py:10: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <_pytest.capture.CaptureFixture object at 0x7fe560f6a358>

    def readouterr(self):
        try:
            return self._capture.readouterr()
        except AttributeError:
>           return self._outerr
E           AttributeError: 'CaptureFixture' object has no attribute '_outerr'

../_pytest/capture.py:210: AttributeError
---------------------------- Captured stdout setup -----------------------------
123 456 789
=========================== 2 error in 0.06 seconds ============================

System information

$ python --version
Python 3.6.2

$ pip freeze
py==1.4.34
-e git://github.com/pytest-dev/pytest.git@539523cfee4c49a765569abcf68134b1255eedb5#egg=pytest

# also happens on 3.2.1

$ uname
Linux

$ uname -m
x86_64

$ head -n2 /etc/os-release 
NAME=Fedora
VERSION="26 (Twenty Six)"

[nicoddemus]

Hi @hroncok thanks for the report!

To me it is a bug; if using capsys from fixtures cannot be supported at least it should provide a better error message.

[ApaDoctor]

@nicoddemus
In the first code _capture method exists with in CaptureFixture all time starting from the begining(function declaration)
In the first example _capture doesn't exist.
_capture method inits in _start.
_start called in CaptureManager
Next steps is hard to discover.
So what the solution of this problem?
Just to correct error message?
What is your recomendation?

[hroncok]

I'm deeply sorry, but I don't understand what are you trying to say. Could you please be a little bit more verbose?

[ApaDoctor]

@hroncok
Oh, this question wasn't for you)
I forgot to add direct mention)

[nicoddemus]

@ApaDoctor I have a working fix, I will prepare a PR later today. Thanks for the interest, your question forced me to study the code and I ended up finding a solution.

[hroncok]

Thanks for fixing this.