python - How to print to console in pytest?

Question

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

But:

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?

Answer 1

By default, 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.

For example,

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 ======================

Note the Captured stdout section.

If you would like to see print statements as they are executed, you can pass the -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 ======================