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Python | Testing Output to stdout

Last Updated : 12 Jun, 2019
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Testing is a critical part of development as there is no compiler to analyze the code before Python executes it.
Given a program that has a method whose output goes to standard Output (sys.stdout). This almost always means that it emits text to the screen. One likes to write a test for the code to prove that, given the proper input, the proper output is displayed.

Using the unittest.mock module’s patch() function, it’s pretty simple to mock out sys.stdout for just a single test, and put it back again, without messy temporary variables or leaking mocked-out state between test cases.

Code #1 : Example

def urlprint(protocol, host, domain):
    url = '{}://{}.{}'.format(protocol, host, domain)

The built-in print function, by default, sends output to sys.stdout. In order to test that output is actually getting there, it is to be mocked out using a stand-in object, and then make assertions about what happened.

Using the unittest.mock module’s patch() method makes it convenient to replace objects only within the context of a running test, returning things to their original state immediately after the test is complete.

Code #2 : Test code for the above code

from io import StringIO
from unittest import TestCase
from unittest.mock import patch
import mymodule
class TestURLPrint(TestCase):
    def test_url_gets_to_stdout(self):
        protocol = 'http'
        host = 'www'
        domain = ''
        expected_url = '{}://{}.{}\n'.format(protocol, host, domain)
        with patch('sys.stdout', new = StringIO()) as fake_out:
            mymodule.urlprint(protocol, host, domain)
            self.assertEqual(fake_out.getvalue(), expected_url)


  • The urlprint() function takes three arguments, and the test starts by setting up dummy arguments for each one. The expected_url variable is set to a string containing the expected output.
  • To run the test, the unittest.mock.patch() function is used as a context manager to replace the value of sys.stdout with a StringIO object as a substitute.
  • The fake_out variable is the mock object that’s created in this process. This can be used inside the body of the with statement to perform various checks. When the with statement completes, patch conveniently puts everything back the way it was before the test ever ran.
  • It’s worth noting that certain C extensions to Python may write directly to standard output, bypassing the setting of sys.stdout.

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