Running In-Process Tests

You might combine some of the coding patterns we’ve just looked at in a test-driver application that tests other code within the same process:

import sys
log = open('testlog', 'a')
from testapi import moreTests, runNextTest, testName
def testdriver():
    while moreTests():
        try:
            runNextTest()
        except:
            print('FAILED', testName(), sys.exc_info()[:2], file=log)
        else:
            print('PASSED', testName(), file=log)
testdriver()

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The testdriver function here cycles through a series of test calls (the module testapi is left abstract in this example). Because an uncaught exception in a test case would normally kill this test driver, you need to wrap test case calls in a try if you want to continue the testing process after a test fails. The empty except catches any uncaught exception generated by a test case as usual, and it uses sys.exc_info to log the exception to a file. The else clause is run when no exception occurs—the test success case.

Such boilerplate code is typical of systems that test functions, modules, and classes by running them in the same process as the test driver. In practice, however, testing can be much more sophisticated than this. For instance, to test external programs, you could instead check status codes or outputs generated by program-launching tools such as os.system and os.popen, covered in the standard library manual (such tools do not generally raise exceptions for errors in the external programs—in fact, the test cases may run in parallel with the test driver).

At the end of this chapter, we’ll also meet some more complete testing frameworks provided by Python, such as doctest and PyUnit, which provide tools for comparing expected outputs with actual results.