def assertIn(self, containee, container, msg=None):
        """
        Fail the test if C{containee} is not found in C{container}.

        @param containee: the value that should be in C{container}
        @param container: a sequence type, or in the case of a mapping type,
                          will follow semantics of 'if key in dict.keys()'
        @param msg: if msg is None, then the failure message will be
                    '%r not in %r' % (first, second)
        """
        if containee not in container:
            raise self.failureException(msg or "%r not in %r"
                                        % (containee, container))
        return containee

        Fail the test if C{containee} is not found in C{container}.

        @param containee: the value that should be in C{container}
        @param container: a sequence type, or in the case of a mapping type,
                          will follow semantics of 'if key in dict.keys()'
        @param msg: if msg is None, then the failure message will be
                    '%r not in %r' % (first, second)
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/_synctest.py