In Python, testing the standard output of a function

Question

I've been trying to write a test (using unittest) to test the output of a function. The function is a follows:

def main():
    arg_pressent = len(sys.argv)
    if arg_pressent < 2:
        print "usage: ./pyPeerChat [IP ADDRESS of pc] / [0 (if the network is not known. This will assume that this peer will be the start of a new network)]"
    else: 
          IP = str(sys.argv[1])
          connect.main(IP)

if __name__ == '__main__':
     main()

So, my test would need test the fact that, when this function runs on its own (not being passed any arguments), it prints 'usage: ./pyPeerChat [IP ADDRESS of pc] / [0 (if the network is not known. This will assume that this peer will be the start of a new network)]'.

So far, the test I've currently been trying to implement has been:

import myModuleChat
from io import StringIO
import unittest
from mock import patch

def main():
     Testmain.test_main_prints_without_ARGs()

class TestMain(unittest.TestCase):
     def test_main_prints_without_ARGs(self):
          expected_print = 'usage: ./pyPeerChat [IP ADDRESS of Bootpeer] / [0 (if the network is not known. This will assume that this peer will be the #start of a new network)]'
          with patch('sys.stdout', new=StringIO()) as fake_out:
               pyPeerChat.main()
          self.assertEqual(fake_out.getvalue(), expected_print)

if __name__ == '__main__':
     test_program = unittest.main(verbosity=0, buffer=False, exit=False)

However, I've been unable to get this test to pass sucessfully. The test fails, and I get an error. Below is the entire output of the test, with the error:

======================================================================
ERROR: test_main_prints_without_ARGs (__main__.TestMain)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "testpyPeerChat.py", line 24, in test_main_prints_without_ARGs
    pyPeerChat.main()
  File "/home/peer-node/Testing/P2PChat/source/pyPeerChat.py", line 47, in main
    print "usage: ./pyPeerChat [IP ADDRESS of Bootpeer] / [0 (if the network is not known. This will assume that this peer will be the start of a new network)]"
TypeError: unicode argument expected, got 'str'

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)

I'm not at all sure what this error means because the code works fine. Is there an easier way to write the test I need, or a way to fix my test?

If you have the rest of that error, it'll tell us the line it happened at. — Celeo
– Celeo, Commented Apr 16, 2015 at 21:33
The indentation has been fixed by someone (thanks!), and I added the entire output of the test, showing how the test failed and the error. — Brewer
– Brewer, Commented Apr 16, 2015 at 21:45

Brewer · Accepted Answer · 2015-04-16 23:07:09Z

Okay, so after a bit more searching, I worked out how to bind the stdout from my function to a variable. Then, using assertEqual(), I was able to compare it to an 'expected' string:

from pyPeerChat import main
from StringIO import StringIO
import unittest


class TestFoo(unittest.TestCase):
    def test_output_without_args(self):
        out = StringIO()
            main(out=out)
            output = out.getvalue().strip()
            expected = 'usage: ./pyPeerChat [IP ADDRESS of Bootpeer] / [0 (if the network is not known. This will assume that this peer will be the start of a new network)]'
        self.assertEqual(output, expected)



if __name__ == '__main__':
    unittest.main() 

#'usage: ./pyPeerChat [IP ADDRESS of Bootpeer] / [0 (if the network is not known. This will assume that this peer will be the start of a new network)]'

This gives me a successfully passed test.

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