How to capture 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. (this
behavior can be configured by the --show-capture
command-line option).
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 three ways in which pytest
can perform capturing:
fd
(file descriptor) 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 filessys.stdout
andsys.stderr
will be captured. No capturing of writes to filedescriptors is performed.tee-sys
capturing: Python writes tosys.stdout
andsys.stderr
will be captured, however the writes will also be passed-through to the actualsys.stdout
andsys.stderr
. This allows output to be ‘live printed’ and captured for plugin use, such as junitxml (new in pytest 5.4).
You can influence output capturing mechanisms from the command line:
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
pytest --capture=tee-sys # combines 'sys' and '-s', capturing sys.stdout/stderr
# and passing it along to the actual sys.stdout/stderr
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", 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:
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-8.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
collected 2 items
test_module.py .F [100%]
================================= FAILURES =================================
________________________________ test_func2 ________________________________
def test_func2():
> assert False
E assert False
test_module.py:12: AssertionError
-------------------------- Captured stdout setup ---------------------------
setting up <function test_func2 at 0xdeadbeef0001>
========================= short test summary info ==========================
FAILED test_module.py::test_func2 - assert False
======================= 1 failed, 1 passed in 0.12s ========================
Accessing captured output from a test function¶
The capsys
, capsysbinary
, capfd
, and capfdbinary
fixtures
allow access to 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")
captured = capsys.readouterr()
assert captured.out == "hello\n"
assert captured.err == "world\n"
print("next")
captured = capsys.readouterr()
assert captured.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
fixture 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).
The return value from readouterr
changed to a namedtuple
with two attributes, out
and err
.
If the code under test writes non-textual data, you can capture this using
the capsysbinary
fixture which instead returns bytes
from
the readouterr
method.
If the code under test writes non-textual data, you can capture this using
the capfdbinary
fixture which instead returns bytes
from
the readouterr
method. The capfdbinary
fixture operates on the
filedescriptor level.
To temporarily disable capture within a test, both capsys
and capfd
have a disabled()
method that can be used
as a context manager, disabling capture inside the with
block:
def test_disabling_capturing(capsys):
print("this output is captured")
with capsys.disabled():
print("output not captured, going directly to sys.stdout")
print("this output is also captured")