Writing a pytest function for checking the output on console (stdout)

Question

This link gives a description how to use pytest for capturing console outputs. I tried on this following simple code, but I get error

import sys
import pytest
def f(name):
    print "hello "+ name

def test_add(capsys):
    f("Tom")
    out,err=capsys.readouterr()
    assert out=="hello Tom"


test_add(sys.stdout)

Output:

python test_pytest.py 
hello Tom
Traceback (most recent call last):
  File "test_pytest.py", line 12, in <module>
    test_add(sys.stdout)
  File "test_pytest.py", line 8, in test_add
    out,err=capsys.readouterr()
AttributeError: 'file' object has no attribute 'readouterr'

what is wrong and what fix needed? thank you

EDIT: As per the comment, I changed capfd, but I still get the same error

import sys
import pytest
def f(name):
    print "hello "+ name

def test_add(capfd):
    f("Tom")
    out,err=capfd.readouterr()
    assert out=="hello Tom"


test_add(sys.stdout)

score 62 · Accepted Answer · edited Aug 05 '19 at 11:23

62

Use the capfd fixture.

Example:

def test_foo(capfd):
    foo()  # Writes "Hello World!" to stdout
    out, err = capfd.readouterr()
    assert out == "Hello World!"

See: http://pytest.org/en/latest/fixture.html for more details

And see: py.test --fixtures for a list of builtin fixtures.

Your example has a few problems. Here is a corrected version:

def f(name):
    print "hello {}".format(name)


def test_f(capfd):
    f("Tom")

    out, err = capfd.readouterr()
    assert out == "hello Tom\n"

Note:

Do not use sys.stdout -- Use the capfd fixture as-is as provided by pytest.
Run the test with: py.test foo.py

Test Run Output:

$ py.test foo.py
====================================================================== test session starts ======================================================================
platform linux2 -- Python 2.7.5 -- pytest-2.4.2
plugins: flakes, cache, pep8, cov
collected 1 items 

foo.py .

=================================================================== 1 passed in 0.01 seconds ====================================================================

Also Note:

You do not need to run your Test Function(s) in your test modules. py.test (The CLI tool and Test Runner) does this for you.

py.test does mainly three things:

Collect your tests
Run your tests
Display statistics and possibly errors

By default py.test looks for (configurable iirc) test_foo.py test modules and test_foo() test functions in your test modules.

edited Aug 05 '19 at 11:23

Lars Blumberg

19,326
11
90
127

answered Dec 10 '13 at 23:46

James Mills

18,669
3
49
62

this did not help. I get same error message. I pasted the code above for your reference – brain storm Dec 10 '13 at 23:53
What version of pytest are you using? Try upgrading. This works as expected and shown in all my unit tests. – James Mills Dec 11 '13 at 00:13
you never call this function `test_f(capfd)` in your code, is that ok? – brain storm Dec 11 '13 at 00:57
The test runner and cli tool ``py.test`` does this for you. It does three things (*mainly*): 1) collect all your tests 2) run all your tests and 3) produces statistics and error output – James Mills Dec 11 '13 at 00:59
1

how does it know which function to run for test? all functions with test_xxx? – brain storm Dec 11 '13 at 01:00
1

Correct. THis is configrable IRRC but by default looks for ``test_foo.py`` modules and ``test_foo()`` function in those test modules. – James Mills Dec 11 '13 at 01:01

score 8 · Answer 2 · edited Jul 20 '20 at 11:20

The problem is with your explicit call of your test function at the very end of your first code snippet block:

test_add(sys.stdout)

You should not do this; it is pytest's job to call your test functions. When it does, it will recognize the name capsys (or capfd, for that matter) and automatically provide a suitable pytest-internal object for you as a call argument. (The example given in the pytest documentation is quite complete as it is.)

That object will provide the required readouterr() function. sys.stdout does not have that function, which is why your program fails.

Writing a pytest function for checking the output on console (stdout)

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