Python Cookbook, 3rd Edition

Chapter 14. Testing, Debugging, and Exceptions

Testing rocks, but debugging? Not so much. The fact that there’s no compiler to analyze your code before Python executes it makes testing a critical part of development. The goal of this chapter is to discuss some common problems related to testing, debugging, and exception handling. It is not meant to be a gentle introduction to test-driven development or the unittest module. Thus, some familiarity with testing concepts is assumed.

14.1. Testing Output Sent to stdout

Problem

You have 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. You’d like to write a test for your code to prove that, given the proper input, the proper output is displayed.

Solution

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.

Consider, as an example, the following function in a module mymodule:

# mymodule.py

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

The built-in print function, by default, sends output to sys.stdout. In order to test that output is actually getting there, you can mock it 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 ...