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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 validate_types(self, types=None):
 """Return a valid list of types. `types` may be a string or a list of strings. If `types` is not supplied, returns the default_types. """

        types = types or self.default_types
        if types is None:
            types = []
        if isinstance(types, six.string_types):
            types = [types]
        return types