Mock patch sys.stdout to StringIO as a decorator

Question

I am trying to mock.patch sys.stdout to StringIO as a decorator to record the output for testing.

As a 'with' statement it works this way:

with mock.patch('sys.stdout', new_callable = StringIO) as recorded_output:
 print('OUTPUT')
 r = recorded_output.getvalue()
print(r)

gets 'OUTPUT', but 'getvalue' doesn't work when used as a decorator:

@mock.patch('sys.stdout', new_callable = StringIO)
def test_stdout(self, recorded_output):
 print('OUTPUT')
 r = recorded_output.getvalue()
 print(r)

gets error: Mock object has no attribute 'getvalue'

Does anyone know how to write this as a decorator?

• David Maze: Pytest provides a set of fixtures to capture stdout already; can you use one of these for your test?
• Viper: I don't know why you get this error (I don't when I try it), but your last print won't work anyway since stdout is still mocked at this point.

Answer 1

There is an example in mock documentation that does exactly what you want (and I bet you already read it according to your code :-))

The example is pretty much like this

from io import StringIO
from unittest.mock import patch
def foo():
 print('Something')
@patch('sys.stdout', new_callable=StringIO)
def test(mock_stdout):
 foo()
 assert mock_stdout.getvalue() == 'Something\n'
test()

But, you are trying to use the print function inside the mock, and it is not a directly call to the sys.stdout. A good way to handle with this issue is to add a mock for the print function as the following example.

from io import StringIO
from unittest.mock import patch, call
@patch('builtins.print')
def test_print(mocked_print):
 print('Something')
 print()
 assert mocked_print.mock_calls == [call('Something'), call()]

But if you want to mix the both approaches, then David Maze proposal's certainly is the better way to handle with it.