def decorate(test, decorator):
    """
    Decorate all test cases in C{test} with C{decorator}.

    C{test} can be a test case or a test suite. If it is a test suite, then the
    structure of the suite is preserved.

    L{decorate} tries to preserve the class of the test suites it finds, but
    assumes the presence of the C{_tests} attribute on the suite.

    @param test: The C{TestCase} or C{TestSuite} to decorate.

    @param decorator: A unary callable used to decorate C{TestCase}s.

    @return: A decorated C{TestCase} or a C{TestSuite} containing decorated
        C{TestCase}s.
    """

    try:
        tests = iter(test)
    except TypeError:
        return decorator(test)

    # At this point, we know that 'test' is a test suite.
    _clearSuite(test)

    for case in tests:
        test.addTest(decorate(case, decorator))
    return test

    Decorate all test cases in C{test} with C{decorator}.

    C{test} can be a test case or a test suite. If it is a test suite, then the
    structure of the suite is preserved.

    L{decorate} tries to preserve the class of the test suites it finds, but
    assumes the presence of the C{_tests} attribute on the suite.

    @param test: The C{TestCase} or C{TestSuite} to decorate.

    @param decorator: A unary callable used to decorate C{TestCase}s.

    @return: A decorated C{TestCase} or a C{TestSuite} containing decorated
        C{TestCase}s.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/_asyncrunner.py