Capturing of the stdout/stderr output

Default stdout/stderr/stdin capturing behaviour

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.

In addition, stdin is set to a “null” object which will fail on attempts to read from it because it is rarely desired to wait for interactive input when running automated tests.

By default capturing is done by intercepting writes to low level file descriptors. This allows to capture output from simple print statements as well as output from a subprocess started by a test.

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.

You can influence output capturing mechanisms from the command line:

py.test -s            # disable all capturing
py.test --capture=sys # replace sys.stdout/stderr with in-mem files
py.test --capture=fd  # also point filedescriptors 1 and 2 to temp file

Using print statements for debugging

One primary benefit of the default capturing of stdout/stderr output is that you can use print statements for debugging:

# content of test_module.py

def setup_function(function):
    print ("setting up %s" % function)

def test_func1():
    assert True

def test_func2():
    assert False

and running this module will show you precisely the output of the failing function and hide the other one:

$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.7.6 -- py-1.4.22 -- pytest-2.6.0
collected 2 items

test_module.py .F

================================= FAILURES =================================
________________________________ test_func2 ________________________________

    def test_func2():
>       assert False
E       assert False

test_module.py:9: AssertionError
-------------------------- Captured stdout setup ---------------------------
setting up <function test_func2 at 0x2abe0d7241b8>
==================== 1 failed, 1 passed in 0.01 seconds ====================

Accessing captured output from a test function

The capsys and capfd fixtures allow to access stdout/stderr output created during test execution. Here is an example test function that performs some output related checks:

def test_myoutput(capsys): # or use "capfd" for fd-level
    print ("hello")
    sys.stderr.write("world\n")
    out, err = capsys.readouterr()
    assert out == "hello\n"
    assert err == "world\n"
    print "next"
    out, err = capsys.readouterr()
    assert out == "next\n"

The readouterr() call snapshots the output so far - and capturing will be continued. After the test function finishes the original streams will be restored. Using capsys this way frees your test from having to care about setting/resetting output streams and also interacts well with pytest’s own per-test capturing.

If you want to capture on filedescriptor level you can use the capfd function argument which offers the exact same interface but allows to also capture output from libraries or subprocesses that directly write to operating system level output streams (FD1 and FD2).