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?

Solution 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 ======================

Solution 2

Short Answer

Use the -s option:

pytest -s

Detailed answer

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: fd, sys or no. 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:

1. file descriptor (FD) level capturing (default): All writes going to the operating system file descriptors 1 and 2 will be captured.

2. 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

Solution 3

Using -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:

1. Insert 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.

2. 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 out and err files in a separate tab and let editor automatically refresh it for you, or do a simple py.test; cat out.txt shell 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 :-).

Solution 4

This is the cleanest way I know to print a single statement to sys.stdout (without artificially making your test fail or enabling the -s option) - you get to see the specific output you want and nothing more:

1. Add the in-built parameter capsys to your test function. (This means that you add capsys to the list of parameters, e.g.

def test_function(existing_parameters, capsys):

3. In your code, simply insert:

with capsys.disabled():
   print("this output will not be captured and go straight to sys.stdout")

See https://buildmedia.readthedocs.org/media/pdf/pytest/latest/pytest.pdf (2.11 How to capture stdout/stderr output).

Solution 5

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)

The 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.

View more solutions

Comments

[U.V.]

i tried version 1. with pytest 3.8.1 unfortunately it only prints the test function block, but not the output from print statements :( any more tricks for this?

[z0r]

Perfect for outputting custom test metrics.

[Tim Boland]

hmm...still doesnt log my print statements

[Mart Van de Ven]

@U.V. - Instead of using the print() function, you should put the variable or message you mean to print after the comma in the assert statement. E.g. assert False, what_are_you will 'print out' the value of what_are_you in the pytest report.

[Sukma Saputra]

It's work for me when need print variable in terminal...

[0 _]

To pass the --capture option in every run of pytest, add the line addopts = --capture=tee-sys in the section [pytest] within the file pytest.ini (documentation).

[pabouk - Ukraine stay strong]

This should be the preferred answer! It seems to work flawlessly without side-effects.

[Karimai]

just in this way I can see my print logs. I did not see the print log by -s option nor by creating a file.

[Bernd Wechner]

It would help if "Add the in-built parameter capsys to your test function." included a little more detail for noobs ... I have no idea what that means off hand.

[CodePrinz]

edit queue is full, can't edit: it is really just def test_something(capsys): ... See docs.pytest.org/en/6.2.x/fixture.html.

[Sacha]

@BerndWechner, thank you for the suggestion, I've edited the answer.