Python – Outputting data from unit test in Python

pythonunit testing

If I'm writing unit tests in Python (using the unittest module), is it possible to output data from a failed test, so I can examine it to help deduce what caused the error?

I am aware of the ability to create a customized message, which can carry some information, but sometimes you might deal with more complex data, that can't easily be represented as a string.

For example, suppose you had a class Foo, and were testing a method bar, using data from a list called testdata:

class TestBar(unittest.TestCase):
    def runTest(self):
        for t1, t2 in testdata:
            f = Foo(t1)
            self.assertEqual(f.bar(t2), 2)

If the test failed, I might want to output t1, t2 and/or f, to see why this particular data resulted in a failure. By output, I mean that the variables can be accessed like any other variables, after the test has been run.

Best Answer

We use the logging module for this.

For example:

import logging
class SomeTest( unittest.TestCase ):
    def testSomething( self ):
        log= logging.getLogger( "SomeTest.testSomething" )
        log.debug( "this= %r", self.this )
        log.debug( "that= %r", self.that )
        # etc.
        self.assertEquals( 3.14, pi )

if __name__ == "__main__":
    logging.basicConfig( stream=sys.stderr )
    logging.getLogger( "SomeTest.testSomething" ).setLevel( logging.DEBUG )
    unittest.main()

That allows us to turn on debugging for specific tests which we know are failing and for which we want additional debugging information.

My preferred method, however, isn't to spend a lot of time on debugging, but spend it writing more fine-grained tests to expose the problem.

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