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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 get_source(self, spec, row):
 """ Sources can be specified as plain strings or as a reference to a column. """

        value = self.get_value({'column': spec.get('source_url_column')}, row)
        if value is not None:
            return value
        return spec.get('source_url')