How do I print to console in pytest?

Question

pytest will not print to the console when I use print. The documentation seems to say that it should work by default.

I am using pytest my_tests.py to run this test:

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?

Maybe stdout is being overwritten. What happens if you use `sys.stdout.write("Test")`? How about `sys.__stdout__.write("Test")`? The latter should always write to the system-defined stdout, which should be the console. If the two commands do different things, then stdout is being changed; if they do the same thing, then the problem is something else. — TheSoundDefense, Jul 07 '14 at 18:41

score 467 · Accepted Answer · answered Jul 07 '14 at 18:55

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

Adding `-v` or `--verbose` flag helped me. Not sure how it works for others without it — Denis, Apr 22 '23 at 21:05

lmiguelvargasf · Answer 2 · 2022-11-15T13:40:38.967

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:

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

You wouldn't happen to know how to also display logging statements? Like logger.warning for example? Just got it: `--log-cli-level=WARNING` will display all WARNING logs — smac89, Aug 04 '22 at 18:28

dmitry_romanov · Answer 3 · 2016-08-05T09:29:08.950

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:

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.
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 :-).

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? — U.V., Sep 29 '18 at 22:15
@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. — Mart Van de Ven, May 25 '20 at 04:52

score 53 · Answer 4 · edited Oct 25 '22 at 01:14

53

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:

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):

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).

edited Oct 25 '22 at 01:14

SkyWalker

28,384
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answered May 18 '21 at 05:59

Sacha

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4

This should be the preferred answer! It seems to work flawlessly without side-effects. – pabouk - Ukraine stay strong Nov 14 '21 at 15:00
2

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. – Karimai Dec 22 '21 at 11:20
edit queue is full, can't edit: it is really just `def test_something(capsys): ... ` See https://docs.pytest.org/en/6.2.x/fixture.html. – CodePrinz Mar 02 '22 at 18:25

score 20 · Answer 5 · answered Aug 06 '16 at 17:27

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.

Perfect for outputting custom test metrics. – z0r Jan 16 '20 at 22:22 — z0r, Jan 16 '20 at 22:22

score 10 · Answer 6 · edited Jun 28 '17 at 19:40

10

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.

edited Jun 28 '17 at 19:40

Jesse W at Z - Given up on SE

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answered Jun 28 '17 at 17:59

Karthik Gomadam Rajagopal

117
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2

It's work for me when need print variable in terminal... – Sukma Saputra Jun 17 '20 at 12:08
2

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](https://docs.pytest.org/en/latest/reference/customize.html#pytest-ini)). – 0 _ Jul 03 '21 at 14:08
It shows `pytest: error: unrecognized arguments: --capture=sys` for pytest 7.0.1 – Smile Oct 13 '22 at 17:37

score 4 · Answer 7 · answered Apr 21 '20 at 14:14

I originally came in here to find how to make PyTest print in VSCode's console while running/debugging the unit test from there. This can be done with the following launch.json configuration. Given .venv the virtual environment folder.

    "version": "0.2.0",
    "configurations": [
        {
            "name": "PyTest",
            "type": "python",
            "request": "launch",
            "stopOnEntry": false,
            "pythonPath": "${config:python.pythonPath}",
            "module": "pytest",
            "args": [
                "-sv"
            ],
            "cwd": "${workspaceRoot}",
            "env": {},
            "envFile": "${workspaceRoot}/.venv",
            "debugOptions": [
                "WaitOnAbnormalExit",
                "WaitOnNormalExit",
                "RedirectOutput"
            ]
        }
    ]
}

score 4 · Answer 8 · answered Jul 23 '21 at 07:44

You can also set this via the Pycharm GUI: go to Run > Edit Configurations. There, select the test you want to enable print statements for and add -s to the Additional Arguments field.

I do it like this, because while I primarily use the Pycharm debugger to debug my pytest functions (i.e. via the GUI), my specific use case also requires me to know what's going on elsewhere in my code and print statements can come in handy for that.

score 3 · Answer 9 · answered Jan 18 '23 at 15:41

3

without artificially making your test fail or enabling the -s option

import warnings

text = "asdf" 

warnings.warn(UserWarning("{}".format(text)))

answered Jan 18 '23 at 15:41

Felipe Berretella

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font: https://docs.pytest.org/en/stable/how-to/capture-warnings.html – Felipe Berretella Jan 18 '23 at 15:42
Very nice, does the job. I wanted a warning (indeed) to be printed out once, at the start of the run, in a certain circumstance. The obvious place to do this is conftest.py, in method `pytest_configure`. But at that point, for example, pytest hasn't yet configured its loggers (and they wouldn't be much help anyway). – mike rodent May 24 '23 at 20:31

score 2 · Answer 10 · answered Mar 07 '22 at 08:15

I generally use this command:-

pytest -v -rs --html=report.html --self-contained-html test_Suite.py -s

the above command also generates a report.html file where all the print statements are getting captured and saved. And the -s at the last will show the print statements into the terminal too.

score 0 · Answer 11 · answered May 25 '22 at 20:14

There are already a lot of good answers, but I want to share why I wasn't able to get logs when I ran pytest.

One important note is that each test case must start with the test_ prefix when you write the functions. Without that, there will be no print statements when using pytest.

As for the command, I use the following one to get very comprehensive logs:

python -m pytest -v -rs <particular_file.py> -s -o log_cli-level=DEBUG

By making sure you have the right function names and using my command, you should be able to see console logs for sure.

score 0 · Answer 12 · answered Aug 23 '22 at 14:11

0

If you are working with pytest.ini, consider using:

[pytest]
...
addopts = --capture=no
...

This works well if you are running your tests from an IDE extension which is based on you pytest.ini

answered Aug 23 '22 at 14:11

Greg7000

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How do I print to console in pytest?

12 Answers12

Detailed answer

Setting capturing methods or disabling capturing

Linked