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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def run(self):
 """Called when a file is changed to re-run the tests with nose."""

        if self.auto_clear:
            os.system('cls' if os.name == 'nt' else 'auto_clear')
        else:
            print
        print 'Running unit tests...'
        if self.auto_clear:
            print
        subprocess.call('nosetests', cwd=self.directory)