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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 list_inputs(self):
 """Return a string listing all the Step's input names and their types. The types are returned in a copy/pastable format, so if the type is `string`, `'string'` (with single quotes) is returned. Returns: str containing all input names and types. """

        doc = []
        for inp, typ in self.input_types.items():
            if isinstance(typ, six.string_types):
                typ = "'{}'".format(typ)
            doc.append('{}: {}'.format(inp, typ))
        return '\n'.join(doc)