Question or problem about Python programming:
I’m trying to use TDD (test-driven development) with pytest.
pytest will not print to the console when I use print.
I am using pytest my_tests.py to run it.
The documentation seems to say that it should work by default: http://pytest.org/latest/capture.html
import myapplication as tum class TestBlogger: @classmethod def setup_class(self): self.user = "alice" self.b = tum.Blogger(self.user) print "This should be printed, but it won't be!" def test_inherit(self): assert issubclass(tum.Blogger, tum.Site) links = self.b.get_links(posts) print len(links) # This won't print either.
Nothing gets printed to my standard output console (just the normal progress and how many many tests passed/failed).
And the script that I’m testing contains print:
class Blogger(Site): get_links(self, posts): print len(posts) # It won't get printed in the test.
In unittest module, everything gets printed by default, which is exactly what I need. However, I wish to use pytest for other reasons.
Does anyone know how to make the print statements get shown?
How to solve the problem:
py.test captures the result of standard out so that it can control how it prints it out. If it didn’t do this, it would spew out a lot of text without the context of what test printed that text.
However, if a test fails, it will include a section in the resulting report that shows what was printed to standard out in that particular test.
def test_good(): for i in range(1000): print(i) def test_bad(): print('this should fail!') assert False
Results in the following output:
>>> py.test tmp.py ============================= test session starts ============================== platform darwin -- Python 2.7.6 -- py-1.4.20 -- pytest-2.5.2 plugins: cache, cov, pep8, xdist collected 2 items tmp.py .F =================================== FAILURES =================================== ___________________________________ test_bad ___________________________________ def test_bad(): print('this should fail!') > assert False E assert False tmp.py:7: AssertionError ------------------------------- Captured stdout -------------------------------- this should fail! ====================== 1 failed, 1 passed in 0.04 seconds ======================
Captured stdout section.
If you would like to see
-s flag to
py.test. However, note that this can sometimes be difficult to parse.
>>> py.test tmp.py -s ============================= test session starts ============================== platform darwin -- Python 2.7.6 -- py-1.4.20 -- pytest-2.5.2 plugins: cache, cov, pep8, xdist collected 2 items tmp.py 0 1 2 3 ... and so on ... 997 998 999 .this should fail! F =================================== FAILURES =================================== ___________________________________ test_bad ___________________________________ def test_bad(): print('this should fail!') > assert False E assert False tmp.py:7: AssertionError ====================== 1 failed, 1 passed in 0.02 seconds ======================
-s option will print output of all functions, which may be too much.
If you need particular output, the doc page you mentioned offers few suggestions:
assert False, "dumb assert to make PyTest print my stuff"at the end of your function, and you will see your output due to failed test.
You have special object passed to you by PyTest, and you can write the output into a file to inspect it later, like
def test_good1(capsys): for i in range(5): print i out, err = capsys.readouterr() open("err.txt", "w").write(err) open("out.txt", "w").write(out)
You can open the
errfiles in a separate tab and let editor automatically refresh it for you, or do a simple
py.test; cat out.txtshell command to run your test.
That is rather hackish way to do stuff, but may be it is the stuff you need: after all, TDD means you mess with stuff and leave it clean and silent when it’s ready :-).
From the docs:
During test execution any output sent to stdout and stderr is captured. If a test or a setup method fails its according captured output will usually be shown along with the failure traceback.
pytest has the option
--capture=method in which
method is per-test capturing method, and could be one of the following:
pytest also has the option
-s which is a shortcut for
--capture=no, and this is the option that will allow you to see your print statements in the console.
pytest --capture=no # show print statements in console pytest -s # equivalent to previous command
Setting capturing methods or disabling capturing
There are two ways in which
pytest can perform capturing:
file descriptor (FD) level capturing (default): All writes going to the operating system file descriptors 1 and 2 will be captured.
sys level capturing: Only writes to Python files sys.stdout and sys.stderr will be captured. No capturing of writes to filedescriptors is performed.
pytest -s # disable all capturing pytest --capture=sys # replace sys.stdout/stderr with in-mem files pytest --capture=fd # also point filedescriptors 1 and 2 to temp file
I needed to print important warning about skipped tests exactly when
PyTest muted literally everything.
I didn’t want to fail a test to send a signal, so I did a hack as follow:
def test_2_YellAboutBrokenAndMutedTests(): import atexit def report(): print C_patch.tidy_text(""" In silent mode PyTest breaks low level stream structure I work with, so I cannot test if my functionality work fine. I skipped corresponding tests. Run `py.test -s` to make sure everything is tested.""") if sys.stdout != sys.__stdout__: atexit.register(report)
atexit module allows me to print stuff after
PyTest released the output streams. The output looks as follow:
============================= test session starts ============================== platform linux2 -- Python 2.7.3, pytest-2.9.2, py-1.4.31, pluggy-0.3.1 rootdir: /media/Storage/henaro/smyth/Alchemist2-git/sources/C_patch, inifile: collected 15 items test_C_patch.py .....ssss....s. ===================== 10 passed, 5 skipped in 0.15 seconds ===================== In silent mode PyTest breaks low level stream structure I work with, so I cannot test if my functionality work fine. I skipped corresponding tests. Run `py.test -s` to make sure everything is tested. ~/.../sources/C_patch$
Message is printed even when
PyTest is in silent mode, and is not printed if you run stuff with
py.test -s, so everything is tested nicely already.
According to the pytest docs,
pytest --capture=sys should work. If you want to capture standard out inside a test, refer to the capsys fixture.