How to print to console in pytest? How to print to console in pytest? python python

How to print to console in pytest?


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-2.5.2plugins: cache, cov, pep8, xdistcollected 2 itemstmp.py .F=================================== FAILURES ===================================___________________________________ test_bad ___________________________________    def test_bad():        print('this should fail!')>       assert FalseE       assert Falsetmp.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-2.5.2plugins: cache, cov, pep8, xdistcollected 2 itemstmp.py 0123... and so on ...997998999.this should fail!F=================================== FAILURES ===================================___________________________________ test_bad ___________________________________    def test_bad():        print('this should fail!')>       assert FalseE       assert Falsetmp.py:7: AssertionError====================== 1 failed, 1 passed in 0.02 seconds ======================


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 consolepytest -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 capturingpytest --capture=sys # replace sys.stdout/stderr with in-mem filespytest --capture=fd  # also point filedescriptors 1 and 2 to temp file


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 :-).


matomo