Python: Write unittest for console print

Function foo prints to console. I want to test the console print. How can I achieve this in python?

Need to test this function, has NO return statement :

def foo(inStr):
   print "hi"+inStr

My test :

def test_foo():
    cmdProcess = subprocess.Popen(foo("test"), stdout=subprocess.PIPE)
    cmdOut = cmdProcess.communicate()[0]
    self.assertEquals("hitest", cmdOut)

You can easily capture standard output by just temporarily redirecting sys.stdout to a StringIO object, as follows:

import StringIO
import sys

def foo(inStr):
    print "hi"+inStr

def test_foo():
    capturedOutput = StringIO.StringIO()          # Create StringIO object
    sys.stdout = capturedOutput                   #  and redirect stdout.
    foo('test')                                   # Call unchanged function.
    sys.stdout = sys.__stdout__                   # Reset redirect.
    print 'Captured', capturedOutput.getvalue()   # Now works as before.

test_foo()

The output of this program is:

Captured hitest

showing that the redirection successfully captured the output and that you were able to restore the output stream to what it was before you began the capture.

Note that the code above in for Python 2.7, as the question indicates. Python 3 is slightly different:

import io
import sys

def foo(inStr):
    print ("hi"+inStr)

def test_foo():
    capturedOutput = io.StringIO()                  # Create StringIO object
    sys.stdout = capturedOutput                     #  and redirect stdout.
    foo('test')                                     # Call function.
    sys.stdout = sys.__stdout__                     # Reset redirect.
    print ('Captured', capturedOutput.getvalue())   # Now works as before.

test_foo()

This Python 3 answer uses unittest.mock. It also uses a reusable helper method assert_stdout, although this helper is specific to the function being tested.

import io
import unittest
import unittest.mock

from .solution import fizzbuzz


class TestFizzBuzz(unittest.TestCase):

    @unittest.mock.patch('sys.stdout', new_callable=io.StringIO)
    def assert_stdout(self, n, expected_output, mock_stdout):
        fizzbuzz(n)
        self.assertEqual(mock_stdout.getvalue(), expected_output)

    def test_only_numbers(self):
        self.assert_stdout(2, '1\n2\n')

Note that the mock_stdout arg is passed automatically by the unittest.mock.patch decorator to the assert_stdout method.

A general-purpose TestStdout class, possibly a mixin, can in principle be derived from the above.

For those using Python ≥3.4, contextlib.redirect_stdout also exists, but it seems to serve no benefit over unittest.mock.patch.

Python: Write unittest for console print

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